In an accumulator having an accumulator housing which is provided with an oil port connected to a pressure piping of equipment, and an inner portion of the accumulator housing being sectioned into a gas chamber filling with gas and a fluid chamber communicating with the oil port by a bellows and a bellows cap, the bellows cap seals the fluid chamber by coming into contact with a seal on the basis of movement in a stroke in the case that a pressure of the pressure piping is decreased, and the seal is retained by a seal retaining portion which is provided in an inner peripheral side of the seal. In the accumulator, a seal diameter of the seal is set to be larger than an effective diameter of the bellows at a position where the seal comes into contact with the bellows cap.