A cap which has a female screw in the inner peripheral surface and which is to be screwed on a port neck part of a container main body holding a predetermined liquid, wherein a plurality of ribs radially extending astride from an inner surface of a top surface part of the cap to an inner surface of a peripheral wall part and integrally projecting inwards are formed on a peripheral edge part of the inner surface part. A female screw is a screw in the form of two threads composed of spiral mated ridges of two threads, for example. The liquid held in the container main body is, for example, a chlorine-based bleach which contains a surface active agent and a sodium hypochlorite. The mated ridges each are 80 to 120 degrees in thread angle θ and 1 to 15% of a valley radius of the female screw in thread height H.