To Demonstrate the Superiority of IMP (1% OPA-15406 Ointment) to the Vehicle in Adult Patients With AD

Inclusion Criteria: 1. Hospitalization status: Outpatient. 2. Age: 15 to 70 years old, inclusive (at time of obtaining informed consent), either male or female (only for 4 weeks double blind treatment). 3. Able to provide written informed consent. For subjects under 18 years old (Korea: 19 years old), written informed consent must be obtained from both the subject and the subject's legal guardian. 4. Diagnosis of AD based on the criteria of Hanifin and Rajka. (See Appendix 1). 5. History of AD for at least 3 years. 6. Atopic dermatitis affecting ≥5% to ≤40% of BSA (excluding scalp) at the screening and baseline visit (only for 4 weeks double blind treatment). 7. IGA score of 2 or 3 at the screening and baseline visit (only for 4 weeks double blind treatment). Exclusion Criteria: 1. Subjects who are pregnant, possibly pregnant, or breastfeeding, who desire to become pregnant or to have their partner become pregnant during the trial period and up until 30 days after the final administration of IMP, or who are unable to either remain abstinent or employ at least two of the specified birth control methods (vasectomy, tubal ligation, vaginal diaphragm, intrauterine device \[IUD\], birth control pill, condom with spermicide, etc.) during the trial period and up until 30 days after the final administration of IMP. 2. Subjects who defined as AD or contact dermatitis rapid deterioration, within 28 days prior to the baseline visit. 3. Subjects who have a concurrent or history of skin disease other than AD (e.g., acne, psoriasis, etc.) and who are judged inappropriate for assessment of AD in the present trial. 4. Subjects who have an active viral skin infection (e.g., herpes simplex, herpes zoster, chicken pox) or clinical signs of such infection. 5. Subjects with a current or history of malignancy within the previous 5 years. 6. Subjects with a current or history of recurrent bacterial infection resulting in hospitalization or requiring intravenous antibiotic treatment within the past 2 years. 7. Subjects with a clinically significant complication or history of any of the following disorders that the investigator judges would prevent safe conduct of the trial or impact efficacy assessment of the IMP: * Cardiac disease (e.g., rheumatic fever or heart valve replacement). * Endocrinologic disease (e.g., severe or uncontrolled diabetes). * Pulmonary disease. * Neurologic disease. * Psychiatric disease. * Hepatic disease (e.g., carriers of hepatitis B, hepatitis C, etc.). * Renal disease. * Hematologic disease. * Immunologic or immunocompromised disease (e.g., acquired immunodeficiency syndrome, Wiskott-Aldrich syndrome, carriers of human immunodeficiency virus \[HIV\] antibodies, etc.). * Other major disease (e.g., systemic fungal infection) or other severe uncontrolled condition (e.g., drug or alcohol abuse) judged by the investigator to pose a health risk to the subject or to have the potential to impact efficacy assessment of the IMP. 8. Subjects with any of the following hematology or serum chemistry results at screening visit: * White blood cell count: ≤3, 000/µL (3 ×109/L) or \>14, 000/µL (14 ×109/L). * Platelets: ≤100, 000/µL (100 ×109/L). * Hemoglobin: \<9 g/dL (90 g/L). * Serum creatinine: ≥2 mg/dL (176.8 μmol/L). * Aspartate aminotransferase (AST) (glutamic oxaloacetic transaminase \[GOT\]): \>2 × ULN. * Alanine aminotransferase (ALT) (glutamic pyruvic transaminase \[GPT\]): \>2× ULN. * Total bilirubin: ≥2.0 mg/dL (34.2 μmol/L). * Any other abnormal laboratory test result that the investigator judges to be a clinically significant abnormality. 9. Subjects who are judged by the investigator to have a clinically significant abnormal blood pressure or pulse rate at the screening and baseline visits. 10. Subjects who are unable to stop allergen immunotherapy (or desensitization therapy) from 3 months prior to providing informed consent until the Week 4 visit (or at the time of withdrawal visit). 11. Subjects who are unable to stop treatment with ultraviolet A, narrowband ultraviolet B, and ultraviolet B from 28 days prior to the baseline examination until the Week 4 visit/ 24 weeks (long-term administration). 12. Subjects who are unable to stop using systemic corticosteroids, systemic immunosuppressant's, systemic antimetabolites, systemic retinoid, and biologics from 28 days prior to the baseline visit until the Week 4 visit/ 24 weeks (long-term administration). 13. Subjects who are unable to stop using topical corticosteroids for skin (excluding scalp) categorized as very strong potency in Appendix 2 'Rank of Topical Corticosteroids' from 21 days prior to the baseline visit until the Week 4 visit/ 24 weeks (long-term administration). 14. Subjects who are unable to stop using topical corticosteroids for skin (excluding scalp) categorized as strong potency in Appendix 2 'Rank of Topical Corticosteroids', topical corticosteroids other than those for skin, topical immunosuppressant's, topical retinoid, topical antihistamine and topical non-steroidal anti-inflammatory drugs (excluding for scalp) from 7 days prior to the baseline visit until the Week 4 visit/ 24 weeks (long-term administration). However, intra-ocular, intra-nasal, intra-auricular, and inhaled corticosteroids may be considered if the investigator judges that their use will not impact assessment of the affected area. 15. Subjects who are unable to stop using topical corticosteroids for skin (excluding scalp) categorized as low or medium potency in Appendix 2 'Rank of Topical Corticosteroids' from 4 days prior to the baseline visit until the Week 4 visit/ 24 weeks (long-term administration). 16. Subjects who are unable to continue in the trial without changing the dosage and administration of systemic antihistamines, sodium cromogicate, tranilast, or suplatast tosilate from 7 days prior to the baseline visit until the Week 4 visit/ 24 weeks (long-term administration). 17. Subjects with known hypersensitivity (including history) to any drugs (prescription, OTC, etc.) or any ingredient of OPA-15406 ointment (e.g., white petrolatum, mineral oil, paraffin, white wax, or propylene carbonate). 18. Subjects with known plans to receive any of the prohibited concomitant drugs or therapies during the trial period. 19. Subjects who have participated in previous trials for OPA-15406 and have been administered the IMP (only for 4 weeks double blind treatment). 20. Subjects who have used any other investigational drug within 4 months prior to the baseline visit or who are scheduled to participate in any other clinical trial during the trial period. 21. Subjects who have never been treated with medication for AD or who are satisfied with their current AD treatment regimen. 22. Subjects who do not respond at all to treatment with existing topical drugs for AD. 23. Subjects who are judged by the investigator to be inappropriate to participate in the trial for any other reason.