The objective of this study was to evaluate the efficacy and safety of stepwise introduction of insulin lispro mix 50 (LM50) from once to 3 times daily in Japanese patients with type 2 diabetes mellitus inadequately controlled by oral therapy. This was a multicenter, open-label, non-randomized trial consisting of three 16-week periods (48 weeks total); all patients were given once-daily injections of LM50 in Period 1. The regimen was intensified to twice daily in Period 2, and 3 times daily in Period 3 if HbA_1c was ≥6.9% before the start of the period. A total of 135 patients were enrolled, and 116 patients completed the study. Main baseline characteristics of enrolled patients were a mean age of 60.3 years, mean diabetes duration of 11.4 years, mean BMI of 25.2 kg/m², and mean HbA_1c of 8.71%. The percentages of patients who achieved HbA_1c levels <6.9% and <7.4% at endpoint were 18.5% (25/135 patients) and 52.6% (71/135 patients), respectively. Mean HbA_1c decreased significantly from 8.70% to 7.44% (p<0.001). The incidence of hypoglycemic episodes over the treatment periods was 65.9% (89/135 patients); severe hypoglycemia occurred in 2.2% (3/135 patients). There were no other clinically significant safety issues related to the study drug. Stepwise introduction of LM50 from once to 3 times daily can be a safe, effective, and simple therapy for Japanese patients with type 2 diabetes mellitus inadequately controlled by oral therapy.