This study examined the association among the onset of diabetic kidney disease (DKD), blood glucose levels (HbA1), and body mass index (BMI) in Japanese patients with type 2 diabetes mellitus.Patients eligible for this study included those with type 2 diabetes who visited the outpatient clinic at Kawasaki Medical School Hospital between 2000 and 2018 and were followed up for more than two years. The Cox proportional hazards model was used in four categories of subjects: at the beginning of the follow-up period, "controlled" or "uncontrolled" glycemic control based on HbA1c and "overweight" or "non-overweight" based on BMI.After dividing the participants into four categories according to HbA1c (lower than 7.0% (C) or higher (U)), and BMI (25 kg/m or higher (O) or lower (N)), hazard ratios for groups CO, UN, and UO were 1.40 (95% CI 1.03-1.90, P = 0.030), 1.40 (1.04-1.88, P = 0.027), and 1.54 (1.12-2.11, P = 0.008), respectively, compared with the CN reference group, after adjustment was made for age, sex, duration of diabetes, and medication for hypertension or dyslipidemia.Maintenance of both an HbA1c level lower than 7.0% and a BMI lower than 25 kg/m was important for the prevention of DKD in Japanese patients with type 2 diabetes mellitus. Both factors had a similar effect on DKD in this study.