A cap to be attached to an extraction port of a container includes a cap main body detached from the container at time of opening, a ring remained on a side on which the container exists at the time of opening, and a passage portion formed separately from the cap main body and provided in the cap main body and the ring. The passage portion includes a leg portion provided in the ring and the cap main body. At least an upper portion of the leg portion provided in the cap main body and a middle portion between the upper portion and a lower portion of the leg portion provided in the ring communicate with each other. When the middle portion of the leg portion is broken, a state of a detection member provided in the passage portion changes.