To provide a resin composition for medical treatments, especially having more improved liquid leakage resistance and resealability, and to provide a stopper for medical treatments, using the same.

This resin composition for the medical treatments comprises (a) 40 to 150 pts.mass of a block copolymer having a mass-average mol. wt. of 50,000 to 550,000 and comprising at least two polymer blocks A consisting mainly of an aromatic vinyl compound and at least one polymer block B consisting mainly of a conjugated diene, and/or a hydrogenated block copolymer obtained by hydrogenating the block copolymer, (b) 1 to 45 pts.mass of a hydrogenated petroleum resin and/or a phenylene ether-based resin, (c) 10 to 60 pts.mass of a peroxide decomposition type olefinic resin, and (d) 40 to 280 pts.mass of a non-aromatic softener for rubbers, and the stopper for medical treatments is formed from the composition.