To provide a cap which can be prevented from being opened mischievously while being displayed or from being removed by catching at capping or packaging for a commercial purpose and is equipped with a function for separate disposal which enables the cap to be ready in a short period of time for recovery for separate disposal.

The cap is such that is made of a resin and forcibly fitted to the edge of an opening of a container. A projecting piece is erected in the axial direction of the cap at a part on the upper outer circumferential part of the cap, and the thickness of the cap at a part extending downward along the axial direction of the cap from the lower ends at both side of the projecting piece is made thin.