To easily confirm from the outside whether a cap has removed when it is removed from the discharge opening.
This cap is provided with a cap body 11 set to the discharge opening and a deformable body 12 provided at the opening side of the cap body 11. The deformable body 12 is formed of a plurality of deformable pieces 13 connected to each other with thin parts 18. Respective deformable pieces 13 are provided with front end parts 14 bents inward through the first folding lines 16. When the cap 10 is removed from the discharge opening 20, the front end parts 14 of the deformable pieces 13 get in contact with a flange 22 of the discharge opening 20. In this case, respective deformable pieces 13 are bent outward through the first folding lines 16 and the second folding lines 17 and the thin parts are broken and the deformable pieces 13 are separated.