Abstract: A blood-collecting device comprises an outer cylinder, a gasket slidable in the outer cylinder, and a plunger for the moving operation of the gasket. A ventilatable filter member is mounted on the tip end part of a hole formed in the gasket. The hole communicated with the outside via the hole of the plunger and the internal space. An anti-coagulation agent supply piece with an anti-coagulation agent such as a heparin supported on one side or both sides of a plate-like supporting member is provided in a space surrounded by the outer cylinder and the gasket. The tip end side of the anti-coagulation agent supply piece having a shape tapered toward the blood inlet opening, and the anti-coagulation agent supply piece maintains its posture by linear contact of a pair of sides facing with each other with the inner surface of the outer cylinder.