The invention provides an accumulator which can reduce the number of the parts of the pressure fluctuation absorption mechanism at the liquid expanding time, can simplify assembling and can reduce a cost of the parts. The accumulator has a seal holder which is provided in a bellows cap, and a plate-like seal which is retained by the seal holder. The seal holder is provided with an inward flange-like outer peripheral side engagement portion, and the seal is provided with an outward projection-like inner peripheral side engagement portion which is arranged in an outer peripheral surface of a seal main body and is made of a rubber-like elastic body engaging with the outer peripheral side engagement portion. The bellows cap moves to a position where liquid pressure and gas pressure are balanced with the seal seating on a seat surface due to difference in pressure receiving area.