To obtain orally administrable antidiabetic drug containing proteoglycan obtained from Grifola frondosa as an active ingredient, effective for treatment and prevention of non-insulin dependent diabetes mellitus.

The objective drug includes proteoglycan of the formula [a number represents a position; (a) and (b) are each a branch; (n) is 100-200] obtained by extracting at least one of Grifola frondosa and Polyporus umbellatus by ≥60°C water and being positive in anthrone reaction and Lowry reaction, having about 2×105 average molecular weight and comprising β-1, 6 bond as a main chain and α-1, 4 branched chain as an essential ingredient. The Grifola frondosa has been ingested as an enable mushroom for a long time and therefore gives no fear of causing side effect. The proteoglycan can be used as it is or by formulating the one with a diluent or a vehicle. The daily dose of the drug is 1-100mg in terms of the proteoglycin and administered by dividing 2-3 times by oral administration, etc.