To provide a bottle cap which can reduce the cost with a less number of parts, and at the same time, can simplify the structure and the motion of a cap automatic fitting device which automatically fits the bottle cap in a bottle mouth section, and also, can prevent the bottom lid from being mistakenly swallowed by a simple structure.

This bottle cap is constituted in such a manner that a bottom lid section 54 is integrally molded on the lower end of an internal cylinder section of a cap main body A, and a notch 54a is formed between the internal cylinder section and the bottom lid section. At the same time, the bottle cap is constituted in such a manner that a bottom lid unsealing section 66 for the bottom lid section may be projected. The bottle cap is also constituted in such a manner that when the internal cylinder section is inserted in a mouth section 50a of a bottle 50 by rotating the cap main body in the sealing direction, the bottom lid unsealing section is closed by the mouth section of the bottle, and is engaged with a throat section at a location having exceeded the throat section 50c in the bottle. The bottle cap is also constituted in such a manner that when the internal cylinder section is removed from the mouth section of the bottle by rotating the cap main body in the unsealing direction, the rising of the bottom lid unsealing section is regulated at the throat section in the bottle, and thus, the bottom lid section is separated from the lower end of the internal cylinder section by the notch, and a raw material C in the internal cylinder section is discharged into the bottle.