A Study to Assess the Efficacy and Safety of ASP1941 in Subjects With Type 2 Diabetes Mellitus Who Have Inadequate Glycemic Control on Metformin and Sitagliptin

Inclusion Criteria: * Subject has been diagnosed with type 2 diabetes mellitus. * Subject has an HbA1c value between 7.0% and 10.5 % at visit 1. * Subject shows FPG (Fasting Plasma Glucose) value is less than 270mg/dL. * Subject shows a BMI of 20.0 to 45.0 kg/m2. * Subject is on stable diet and exercise program at least 8 weeks prior to study participation. * Postmenopausal female or surgically sterile female or agree not to become pregnant. Exclusion Criteria: * Subject has type 1 diabetes mellitus. * Subject has proliferative diabetic retinopathy. * Subject has a history of clinically significant renal disease(s) such as renovascular occlusive disease, nephrectomy, or renal transplant. * Subject has significant dysuria caused by a neurogenic bladder or a benign prostatic hypertrophy etc. * Subject has a symptomatic urinary tract infection or genital infection. * Subject has chronic disease(s) that require the continuous use of systemic corticosteroids or immunosuppressants. * Subject has cardiovascular disease or cerebrovascular disease that may affect the administration of ASP1941 or its safety assessment in the opinion of the investigator or sub-investigator. * Subject has uncontrollable psychiatric disorder(s) with medication. * Male subject is not willing to use appropriate contraception during the study. * Subject has severe infection, serious trauma, or perioperative subject. * Subject has a malignant tumor or has a history of malignant tumor. * Subject has severe gastrointestinal disease, or history of operation for serious gastrointestinal disease. * Subject has diabetic ketoacidosis. * Subject has lactic acidosis or has history of lactic acidosis. * Subject has hepatitis or is a carrier of hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) antibody, or is positive for HIV-1 and/or HIV-2. * Subject has a history of thyroid dysfunction and acute pancreatitis. * Subject is on weight-loss program or weight-loss medication (e.g. orlistat, phentermine/topiramate, lorcaserin) within 12 weeks of study participation. * Subject has a history of serious cardiac diseases (NYHA Class III to IV), congestive heart failure, arrhythmia in need of medical treatment. * Subject has a history of cerebrovascular attack, unstable angina, myocardial infarction, angioplasty, within 52 weeks (364 days) of study participation. * Subject has uncontrollable severe hypertension, i.e., systolic blood pressure of higher than 180 mmHg or diastolic blood pressure of higher than 110mmHg measured in a sitting position after a 5-minute rest. * Subject has following AST or ALT value: * AST: Male \> 100 U/L, Female \> 80 U/L * ALT: Male \> 102.5 U/L, Female \> 82.5 U/L * Subject has following serum creatinine value: * Creatinine: Male \> 1.3 mg/dL, Female \> 1.2 mg/dL * Subject has estimated GFR values, of \< 60 mL/min/1.73m2.