The present invention relates to a Type II diabetes model mouse, having a causative gene responsible for the following characteristics (1) to (3) in the heterologous or homologous state, wherein the characteristics are autosomal recessive genetic traits: (1) having a higher blood sugar level as compared to a normal strain mouse at 10 to 14 weeks of age and having a blood glycosylated hemoglobin concentration of 2.5% or higher at 10 to 14 weeks of age or older; (2) being positive in test for urine sugar at 10 to 14 weeks of age; and (3) exhibiting essentially no inflammation of the pancreatic islets and having a blood insulin concentration equivalent to or higher than that of a normal strain mouse. According to the present invention, there is provided a novel mouse having a gene responsible for the spontaneous development of type II diabetes.