To restrain abnormal deformation of a bellows due to a pressure difference between the inside and outside thereof, an outside gas type accumulator includes a pressure difference regulation mechanism (21) having a movable plate (22) supported by a coil spring (23) on the oil port (4) side of a bellows cap (8), the plate (22) moves together with the cap (8) in a state of being supported by the spring (23), during normal operation, the plate (22) moves together with the cap (8) to be brought into contact with a seal (13) at zero-down time, and the plate (22) keeps contact with the seal (13) and the cap (8) moves to a position where liquid pressure and gas pressure balances while compressing the spring (23) when the liquid and the charged gas expand thermally.