An outer gas type or inner gas type accumulator has a mechanism for reducing a pressure difference generated when liquid in a liquid chamber and gas expand thermally at a zero-down time, the pressure difference between the inside and outside of a bellows is reduced to suppress abnormal deformation of the bellows, a movable plate supported by a spring is provided at the bellows cap side of an oil port, the movable plate is supported by the spring and away from a seal in normal operation, the movable plate is pushed by the bellows cap and contacts with the seal while elastically deforming the spring, in a zero-down state, and when the liquid and the gas expand thermally in the zero-down state, the bellows cap moves to a position where the liquid pressure and the gas pressure balance, while the movable plate keeps contact with the seal.