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.

The cap consists of a cap main body (11) and a tamperproof ring (12) fitted to the inside of the cap main body. The ring (12) has upper and lower two step ring-like shapes, which are respectively divided into two or more. The upper ring (13) is formed smaller than the lower ring (14), and divided respective upper ring and lower ring are integrally connected with each other with bridge entangling pieces (15). An upper ratchet piece (131) is formed on the outer peripheral face of the upper ring, and a lower ratchet piece (141) to be engaged on a cylindrical part projection (22) formed on the outer peripheral face of a cylindrical part (21) is formed on the inner peripheral face of the lower ring. A cap ratchet piece (111) to be engaged with the upper ratchet piece is formed on the inner peripheral face of the cap main body.