An outside gas type accumulator has a mechanism for reducing the pressure difference occurring when liquid confined in a liquid chamber and gas filling the liquid chamber in zero-down state expand thermally, and restrain abnormal deformation of a bellows by reducing the pressure difference between the inside and outside of the bellows. A pressure difference regulation mechanism (21) reduces the pressure difference occurring when liquid confined in the liquid chamber (11) and gas filling the liquid chamber in the zero-down state expand thermally. The regulation mechanism (21) has a movable plate (22) supported by a coil spring (23) on the oil port (4) side of a bellows cap (8) . During normal operation, the plate (22) moves together with the cap (8) by being supported by the spring (23), and the plate (22) moves together with the cap (8) and to be brought into contact with a seal (13) in the zero-down state. When the liquid and the filling gas expand thermally, the plate (22) maintains contact with the seal (13), and the cap (8) moves up to a position where the liquid pressure and the gas pressure are balanced while compressing the spring (23).