A plastic cap of this invention is opened by tearing-off and is comprising a top plate (1) provided with a sealing portion to be engaged with the mouth of a container, a skirt (2) extending perpendicularly from the top plate, and a tear-off portion (3) arranged around the skirt and defining an upper closure portion for resealing and a lower opening band, wherein in the inner surface of the skirt, at an upper position than the tear-off portion, a diametrically inwardly-directed protrusion which is engaged with a jaw of the container is formed peripherally intermittently, simultaneously, at a lower position than the tear-off portion, a flap piece (7) whose tip portion (71) is engaged with the jaw of the bottle is formed peripherally intermittently, and the engaged protrusion (6) and the flap piece are formed at least alternately in most of the peripheral direction. The plastic cap has excellent sealing reliability, easy openability, and tamper-evident property while the cap height is lowered and the resin weight of the resin is reduced.