An accumulator comprising: an accumulator housing which is provided with an oil port connectable to a pressure piping of an equipment; an inner portion of the accumulator housing being sectioned into a gas chamber filled with gas and a fluid chamber communicating with said oil port by a bellows and a bellows cap in a discoid shape, wherein the fluid chamber is outward of a seal and is set in an outer peripheral side of the bellows; said bellows cap sealing said fluid chamber from said oil port by coming into contact with the seal on the basis of movement in a stroke in the case that a pressure of said pressure piping is decreased; and in the accumulator housing said seal being installed to an inner surface of the housing and being retained by a seal retaining portion which is formed in the inner surface of the housing and is provided at an inner peripheral side of the seal, said seal being formed in annular shape and having one single seal lip having a lip end coming into contact with said bellows cap, wherein a seal diameter of said seal lip end is set to be larger than an effective diameter of said bellows at a position where said seal comes into contact with said bellows cap, wherein the seal diameter corresponds to a position at which said seal lip end comes into contact with said bellows cap, and wherein the effective diameter is determined according to the following expression d 2 = √ 4 ∗ V / π ∗ S wherein S is set to a stroke of the bellows from a state in which the accumulator accumulates pressure, to a state in which the bellows is extended due to the pressure decrease of the piping, and V is set to a liquid discharge amount of the liquid supplied to the piping from the accumulator, by the state change from the state in which the accumulator accumulates the pressure to the state in which the bellows is expanded due to the pressure decrease of the piping. an accumulator housing which is provided with an oil port connectable to a pressure piping of an equipment; an inner portion of the accumulator housing being sectioned into a gas chamber filled with gas and a fluid chamber communicating with said oil port by a bellows and a bellows cap in a discoid shape, wherein the fluid chamber is outward of a seal and is set in an outer peripheral side of the bellows; said bellows cap sealing said fluid chamber from said oil port by coming into contact with the seal on the basis of movement in a stroke in the case that a pressure of said pressure piping is decreased; and in the accumulator housing said seal being installed to an inner surface of the housing and being retained by a seal retaining portion which is formed in the inner surface of the housing and is provided at an inner peripheral side of the seal, said seal being formed in annular shape and having one single seal lip having a lip end coming into contact with said bellows cap, wherein a seal diameter of said seal lip end is set to be larger than an effective diameter of said bellows at a position where said seal comes into contact with said bellows cap, wherein the seal diameter corresponds to a position at which said seal lip end comes into contact with said bellows cap, and wherein the effective diameter is determined according to the following expression d 2 = √ 4 ∗ V / π ∗ S wherein S is set to a stroke of the bellows from a state in which the accumulator accumulates pressure, to a state in which the bellows is extended due to the pressure decrease of the piping, and V is set to a liquid discharge amount of the liquid supplied to the piping from the accumulator, by the state change from the state in which the accumulator accumulates the pressure to the state in which the bellows is expanded due to the pressure decrease of the piping.