Primary Objectives: To assess the effects of tofogliflozin on glycemic control in comparison to placebo as an add-on treatment to insulin treatment in terms of glycated hemoglobin (HbA1c) reduction over a period of 16 weeks in patients with type 2 diabetes mellitus. To assess the safety of tofogliflozin in combination with insulin treatment throughout 52 weeks. Secondary Objectives: To assess the effects of tofogliflozin in comparison to placebo on: * Body weight * Fasting plasma glucose (FPG) * Postprandial plasma glucose (PPG) To assess the long term safety and tolerability of tofogliflozin.

The total study duration from screening for a patient can be approximately up to 1 year, including a screening period of 2 weeks, double-blinded placebo controlled treatment period of 16 weeks, an open-labeled extension period of 36 weeks, and a follow-up period of 3 days.