To provide a one-touch cap for preventing it properly that the contents gradually deposit on the portion where a cap and a container mouth fit on each other, to obstruct the fitting between the cap and the container mouth.

An inner cylindrical portion 3, in which a container mouth 10 fits on the top plate rear face 1a of a cap, is vertically disposed, and a through hole 3d for releasing a residual content RC having stuck to a container mouth portion 10b, the top plate rear face 1a and so on, to the outer side of the inner cylindrical portion 3. Here, no special restriction is made on the sectional shape along the side wall portion 3b, the opening area, and the number of the through holes 3d and the set positions from the top plate rear face 1a when the cap 100 is fitted in the container mouth 10, so long as the cap 100 releases the residual content RC to the outside of the inner cylindrical portion 3 whereas an inner circumference protrusion 3a of the inner cylindrical portion 3 is caught by an outer circumference protrusion 10a of the container mouth 10.