Purpose: This study aimed to evaluate the association between a body shape index (ABSI) and incident type 2 diabetes and to explore the shape of their relationship in a cohort of Japanese adults. Patients and Methods: Data from 15,462 Japanese adults aged 18-79 years attending the NAGALA study (NAfld in the Gifu Area, Longitudinal Analysis) were used. Body weight, height, and waist circumference were measured. Blood samples were measured for serum lipid, glucose, and HbA1c. The risk of incident type 2 diabetes according to ABSI was estimated using multivariate Cox regression models. We examined a potential nonlinear relationship using a smoothing function analysis. Subgroup analyses were conducted according to age, gender, smoking status, alcohol intake, fatty liver, and BMI. Results: After adjusting for potential confounding factors (age, gender, smoking status, alcohol intake, fatty liver, systolic blood pressure, BMI, fasting plasma glucose, HbA1c, HDL-cholesterol, triglycerides), a linear relationship was observed between ABSI and risk of type 2 diabetes. The hazard ratio (HR) and 95% confidence intervals (95% CI) for incident type 2 diabetes with ABSI (10(-2) m(11/6)kg(-2/3)) were 1.51 (1.13, 2.01) (p=0.005). When ABSI was handled as categorical variable, the HRs and 95% CIs in the quartile 2 to 4 versus the quartile 1 were 0.97 (0.67, 1.41), 1.21 (0.85, 1.72) and 1.30 (0.92, 1.83), respectively (P for trend = 0.046). Subgroup analyses showed that the association stably existed in different subgroups including gender, age, smoking status, alcohol intake, fatty liver, and BMI. Conclusion: ABSI was linearly associated with an elevated risk of incident type 2 diabetes across the full range of ABSI, independent of gender, age, smoking status, alcohol intake, fatty liver, SBP, BMI, FPG, HbA1c, HDL-cholesterol, and triglycerides.