An accumulator comprising: a housing having a sealing face and a fluid inlet/outlet passage; a bellows fixed at least one end to the housing such that an inner space of the housing is hermetically partitioned by the bellows into an interior and an exterior of the bellows, the bellows including a bellows main body capable of expanding and contracting and a bellows cap including an annular seal holder; and a sealing member formed by covering a disc-shaped substrate with an elastic body that is opposed to and capable of being closely attached to the sealing face of the housing, the sealing member is held by a holding portion of the annular seal holder on an inner diameter side of the annular seal holder, the fluid inlet/outlet passage of the housing being closed upon a close attachment of the elastic body to the sealing face, wherein a through hole passing through in a radial direction is provided in the seal holder, and a communication passage extending in the radial direction so as to partially form a space providing communication between the through hole and the fluid inlet/outlet passage is provided in the seal holder or the sealing face characterized in that the seal holder is formed in a substantially-U shape in a sectional view so as to include a vertical portion extending downward from a fixed portion, a bottom portion extending to the inner diameter side from a lower end of the vertical portion, and a standing portion standing upward from an inner diameter side end portion of the bottom portion, the standing portion being formed in a reversed L shape in a sectional view and holding the sealing member, the standing portion on the radially inward side of the substantially-U shape holding the sealing member, the substrate has a diameter larger than an inner diameter of the standing portion of the seal holder, and the communication passage is a communication hole passing through the standing portion of the seal holder in the radial direction. a housing having a sealing face and a fluid inlet/outlet passage; a bellows fixed at least one end to the housing such that an inner space of the housing is hermetically partitioned by the bellows into an interior and an exterior of the bellows, the bellows including a bellows main body capable of expanding and contracting and a bellows cap including an annular seal holder; and a sealing member formed by covering a disc-shaped substrate with an elastic body that is opposed to and capable of being closely attached to the sealing face of the housing, the sealing member is held by a holding portion of the annular seal holder on an inner diameter side of the annular seal holder, the fluid inlet/outlet passage of the housing being closed upon a close attachment of the elastic body to the sealing face, wherein a through hole passing through in a radial direction is provided in the seal holder, and a communication passage extending in the radial direction so as to partially form a space providing communication between the through hole and the fluid inlet/outlet passage is provided in the seal holder or the sealing face characterized in that the seal holder is formed in a substantially-U shape in a sectional view so as to include a vertical portion extending downward from a fixed portion, a bottom portion extending to the inner diameter side from a lower end of the vertical portion, and a standing portion standing upward from an inner diameter side end portion of the bottom portion, the standing portion being formed in a reversed L shape in a sectional view and holding the sealing member, the standing portion on the radially inward side of the substantially-U shape holding the sealing member, the substrate has a diameter larger than an inner diameter of the standing portion of the seal holder, and the communication passage is a communication hole passing through the standing portion of the seal holder in the radial direction.