An accumulator comprising: a shell inside which an air chamber and a fluid chamber are formed; a bellows which is housed to freely expand and contract in the shell; a port part which is provided on the shell and in which a pressure fluid inflow port that is open to the fluid chamber is formed; and a self-sealing member which is arranged on a tip portion of the bellows to face the port part, wherein when a pressure in the fluid chamber is lower than a predetermined pressure relative to a pressure in the air chamber, the self-sealing member is allowed to abut onto a seal area of the port part to block the pressure fluid inflow port, and wherein the self-sealing member includes a base material part, a resilient member which is arranged on a surface of the base material part to face the seal area, and an overhanging portion which is formed on the resilient member and allowed to abut onto the seal area for sealing, and the port part includes a seal face which is allowed to abut onto the overhanging portion and on which the seal area is formed, and a projecting portion which is arranged on a region other than the seal area on the seal face, and wherein when a temperature in the air chamber rises more excessively than in a normal operation, the overhanging portion on the resilient member transubstantiates and a gap portion is formed which leads from an outer border of the seal face to the pressure fluid inflow port; characterised in that, the bellows partitions the inside of the shell into the air chamber and the fluid chamber; and when the overhanging portion on the resilient member transubstantiates, the projecting portion is allowed to abut onto the base material part and to form the gap portion between the seal face and the self-sealing member so as to release the pressure in the air chamber through the gap portion. a shell inside which an air chamber and a fluid chamber are formed; a bellows which is housed to freely expand and contract in the shell; a port part which is provided on the shell and in which a pressure fluid inflow port that is open to the fluid chamber is formed; and a self-sealing member which is arranged on a tip portion of the bellows to face the port part, wherein when a pressure in the fluid chamber is lower than a predetermined pressure relative to a pressure in the air chamber, the self-sealing member is allowed to abut onto a seal area of the port part to block the pressure fluid inflow port, and wherein the self-sealing member includes a base material part, a resilient member which is arranged on a surface of the base material part to face the seal area, and an overhanging portion which is formed on the resilient member and allowed to abut onto the seal area for sealing, and the port part includes a seal face which is allowed to abut onto the overhanging portion and on which the seal area is formed, and a projecting portion which is arranged on a region other than the seal area on the seal face, and wherein when a temperature in the air chamber rises more excessively than in a normal operation, the overhanging portion on the resilient member transubstantiates and a gap portion is formed which leads from an outer border of the seal face to the pressure fluid inflow port; characterised in that, the bellows partitions the inside of the shell into the air chamber and the fluid chamber; and when the overhanging portion on the resilient member transubstantiates, the projecting portion is allowed to abut onto the base material part and to form the gap portion between the seal face and the self-sealing member so as to release the pressure in the air chamber through the gap portion.