Disclosed is a plastic cap useful for steadfastly maintaining a sealed state created in a container having a mouth with an opening by sealing the opening with a sealing member. The cap has a cap main body and a cover portion integrated with the main body via at least one bridge portion. The main body has a base portion and a top wall. The bridge portion is formed of a synthetic plastic filled in at least one fusion-bonding hole having a diameter of from 0.3 to 3 mm and arranged extending through at least one, preferably one of the main body or cover portion such that the main body and cover portion are integrally fusion-bonded by the bridge portion and the cover portion can be separated from the main body by breaking the bridge portion with hands. A production method of the plastic cap is also disclosed.