A hydraulic accumulator comprising: a shell including an internal space having a gas chamber to be filled with a pressurized gas and a fluid chamber to be filled with a hydraulic-fluid; a bellows that is extendable and contractible, installed in the shell and partitions the internal space into the gas chamber and the fluid chamber, the bellows including a bellows cap at an end portion of the bellows; a port portion provided to the shell and defining a hydraulic-fluid inlet communicating with the fluid chamber; and a self-seal stay disposed in the fluid chamber and comprising a cylindrical body portion surrounding the port portion; and a lid-shaped head portion defining a through hole and extending from the cylindrical body portion, the through hole being configured to be closed by the bellows cap which comes into contact with the head portion; the cylindrical body portion including a first recessed portion recessed from an outer circumferential surface of the cylindrical body portion, extending in a circumferential direction of the cylindrical body portion, and including a bottom surface that is plane or circular arc curved shaped, so that the first recessed portion deforms when a gas pressure in the gas chamber is higher than a predetermined threshold; the hydraulic accumulator characterized by: at least one of both end portions of the bottom surface in the circumferential direction joined with the outer circumferential surface of the cylindrical body portion; and the first recessed portion having a wall thickness which increases from a central portion of the first recessed portion to said at least one of the end portions of the bottom surface in the circumferential direction of the cylindrical body portion. a shell including an internal space having a gas chamber to be filled with a pressurized gas and a fluid chamber to be filled with a hydraulic-fluid; a bellows that is extendable and contractible, installed in the shell and partitions the internal space into the gas chamber and the fluid chamber, the bellows including a bellows cap at an end portion of the bellows; a port portion provided to the shell and defining a hydraulic-fluid inlet communicating with the fluid chamber; and a self-seal stay disposed in the fluid chamber and comprising a cylindrical body portion surrounding the port portion; and a lid-shaped head portion defining a through hole and extending from the cylindrical body portion, the through hole being configured to be closed by the bellows cap which comes into contact with the head portion; the cylindrical body portion including a first recessed portion recessed from an outer circumferential surface of the cylindrical body portion, extending in a circumferential direction of the cylindrical body portion, and including a bottom surface that is plane or circular arc curved shaped, so that the first recessed portion deforms when a gas pressure in the gas chamber is higher than a predetermined threshold; the hydraulic accumulator characterized by: at least one of both end portions of the bottom surface in the circumferential direction joined with the outer circumferential surface of the cylindrical body portion; and the first recessed portion having a wall thickness which increases from a central portion of the first recessed portion to said at least one of the end portions of the bottom surface in the circumferential direction of the cylindrical body portion. at least one of both end portions of the bottom surface in the circumferential direction joined with the outer circumferential surface of the cylindrical body portion; and the first recessed portion having a wall thickness which increases from a central portion of the first recessed portion to said at least one of the end portions of the bottom surface in the circumferential direction of the cylindrical body portion.