This is a randomized, 2-arm, parallel group, open-label study to evaluate the safety and efficacy of TA-7284 in Japanese patients with Type 2 diabetes mellitus, who are not optimally controlled on diet and exercise or with oral anti-hyperglycaemic agent. The patients will receive either TA-7284-Low or TA-7284-High orally alone or in combination with oral anit-hyperglycaemic agent for 52 weeks.