Determination of the incidence rate of diabetic ketoacidosis (DKA) in type 2 diabetes mellitus (T2DM) is urgent, in response to the safety issue with use of the glucose lowering drugs, sodium-glucose cotransporter-2 inhibitors, and DKA.We extracted data of adult patients with T2DM from a medical claims database in Japan, which included 1 million individuals. The study period was 2005-2013, before the era of sodium-glucose cotransporter-2 inhibitors. The inclusion criteria were patients with a diagnosis of T2DM who had at least 1 prescription for a glucose-lowering drug. We further examined the number of DKA-related admissions and medication use prior to admission. Calculation of DKA incidence was on a patient-years basis, with 95% confidence interval using a Poisson distribution.Of 36,674 adult patients with T2DM, we identified 74 cases admitted for DKA, with an estimated DKA incidence of 0.48/1000 (95% confidence interval, 0.38-0.60/1000) patient-years. Of these 74 cases, approximately two-thirds of patients had no past or recent history of glucose-lowering drug use before admission for DKA.This study estimated that the incidence of DKA was 0.48/1000 patient-years in Japan, similar to previous studies. We also found that underuse of glucose-lowering medication was common among patients hospitalized for DKA. KEY POINTS The population-based incidence of DKA in T2DM is largely unknown. In Japan, the estimated DKA incidence was 0.48/1000 (95% CI, 0.38-0.60/1000) patient-years. Drug-induced DKA is an emerging research area, and our results can be applied to evaluating the risk of DKA.