In order to examine the influence of obesity on metabolic disorder and liver pathogenesis of the Fatty Liver Shionogi (FLS) mouse, which develops hereditary fatty liver and spontaneous liver tumors, we established a new congenic strain named FLS-Lep^ob. The Lep^ob gene of the C57BL/6JWakShi (B6)-Lep^ob/Lep^ob mouse was transferred into the genome of the FLS mouse, by backcross mating. FLS-Lep^ob/Lep^ob mice were maintained by intercrossing between Lep^ob-heterozygous littermates. The FLS-Lep^ob/Lep^ob mice of both sexes developed remarkable hyperphagia, obesity and type 2 diabetes mellitus. At 12 weeks of age, glucosuria was detected in all male and female FLS-Lep^ob/Lep^ob mice. Biochemical examination demonstrated that the FLS-Lep^ob/Lep^ob mice have severe hyperlipidemia and hyperinsulinemia. The livers of FLS-Lep^ob/Lep^ob mice showed microvesicular steatosis and deposition of large lipid droplets in hepatocytes throughout the lobules. The steatohepatitis-like lesions including the multifocal mononuclear cell infiltration and clusters of foamy cells were observed earlier in FLS-Lep^ob/ Lep^ob mice than in FLS mice. B6-Lep^ob/Lep^ob mice did not show hepatic inflammatory change. Furthermore, FLS-Lep^ob/Lep^ob mice developed multiple hepatic tumors including hepatocellular adenomas and carcinomas following steatohepatitis. In conclusion, the FLS-Lep^ob/Lep^ob mice developed steatohepatitis and hepatic tumors following hepatic steatosis. The FLS-Lep^ob/Lep^ob mouse with obesity and type 2 diabetes mellitus might be a useful animal model for human non-alcoholic steatohepatitis (NASH).