To provide a tamperproof cap which enables the clear recognition that the cap is opened when the cap is once opened, even though the cap is mounted again on a container, and facilitates discard by classification.

A screw type cap screwed on a cylindrical part (21) of a bung hole main body (20) consists of a cap main body (11) and a tamperproof ring (12) connected to the lower part of the cap main body with an easily separable connecting part (13). The tamperproof ring is provided with a holding part (14) for separating the tamperproof ring from the cap main body by a hand. The holding part is formed by a hole or a knob shape.