OBJECTIVE Information on the clinical efficacy of SGLT2 inhibitors in the Japanese population is limited. The aim of this single-arm, single-center, open-label study was to confirm the body weight- and fat mass-lowering effects of canagliflozin (CANA) and the accompanying improvement in insulin resistance in Japanese patients with Type 2 diabetes mellitus (T2DM). METHODS Thirty-eight patients were enrolled and administered 100 mg CANA once daily for 24 weeks. Blood and anthropometric parameters were examined before and after treatment. In a subset of patients, insulin sensitivity was assessed based on the glucose infusion rate (GIR) during a hyperinsulinemic euglycemic clamp test. RESULTS CANA treatment significantly decreased hemoglobin A1c, fasting plasma glucose, and plasma liver enzyme levels, and increased plasma adiponectin levels. In addition, a significant reduction in body weight, visceral and subcutaneous fat area, fat and lean mass, and liver steatosis was also observed. The change in plasma adiponectin levels significantly correlated with the changes in both body fat mass and visceral fat area. GIR increased from 3.25 ± 1.53 to 4.11 ± 1.30 mg/kg/min (P < 0.05). CONCLUSIONS CANA improved insulin resistance and decreased visceral fat mass in Japanese patients with T2DM.