A medicinal composition which comprises an insulin resistance-improving drug and vitamin B1 or derivative thereof. In this medicinal composition, the side effects of the insulin resistance-improving drug such as edema, heart enlargement, anemia, etc. are prevented by using vitamin B1 or its derivative together. It is usable as a remedy for diabetes, a remedy for lifestyle related diseases, an anti-tumor agent, an anti-rheumatoid drug and so on.