Antidiabetic effects of the acid hydrolysate of silk fibroin were investigated by oral administration to animal models for diabetes mellitus, Fibroin protein was extracted from cocoon and digested to peptides of low-molecular weight range (mainly below 3,000) and amino acids by acid hydrolysis, Feeding of the fibroin hydrolysate resulted in a significant recovering effect on reduction of body weight gain and a lowering effect on blood glucose gain in streptozotocin-induced diabetic Sprague Dawley rats (STZ rats) which were used as an insulin-dependent diabetic animal model. But the body weight and blood glucose level in C57BL/KsJ-db/db mice (db/db mice), an non-insulin-dependent diabetic animal model, were not changed significantly by the feeding, On the other hand, plasma leptin levels increased according to increased feeding amount of the hydrolysate in STZ rats and db/db mice in common, It was concluded from the results that the fibroin hydrolysate might stimulate the insulin secretion by recovering or activating pancreatic ${eta}$ cells and result in the increased plasma leptin level. It was also deduced that the antidiabetic improvements in body weight and blood glucose gain in STZ were thought to be due to the increased insulin secretion, but in db/db mice of which the diabetic symptoms were caused by insulin resistance, the stimulated secretion of insulin was unlikely to be able to change body weight and blood glucose level significantly.