An accumulator has a seal member retained to a port hole side of a bellows cap via a seal holder. The seal member comes into contact with a seal portion so as to occlude a liquid chamber in the case that an operation of a device stops and the pressure within a pressure piping is lowered. The seal member moves in a direction that the bellows cap moves away from the seal portion while being in contact with the seal portion when the liquid confined in the liquid chamber thermally expands in a state in which the liquid chamber is occluded. The seal member is obtained by attaching a flexible portion constructed by a rubber-like elastic body to an outer peripheral surface of a rigid plate, and the flexible portion allows relative movement of the bellows cap by shear deformation on the basis of engagement with the seal holder.