To provide a piston cup having a variable outer peripheral portion with a variable outside diameter slid in accordance with to the inside diameter of a cylinder, and absorbing a difference in the inside diameter from the cylinder.

The piston cup 3 is composed of an elastic body fitted to the end of a plunger piston for a carburetor of an automobile. A circumferential protrusion (projecting ring portion) B is formed at the outer peripheral portion of the piston cup 3, a circulation groove (groove portion) C is formed at the inner peripheral portion of the piston cup with the protrusion positioned, and the outer peripheral portion forms the variable outer peripheral portion. When the difference in the inside diameter from the cylinder is 0.3 mm or smaller, the variable outer peripheral portion slides without a space.