An accumulator comprising: an accumulator housing which has a port hole connectable to a pressure piping of a device; a bellows and a bellows cap which is arranged in an inner portion of said housing so as to partition an internal space of said housing into a gas chamber to which a high-pressure gas is charged and a liquid chamber which is communicated with said port hole; and a seal member which is retained to the port hole side of said bellows cap via a seal holder, wherein said seal member moves together with said bellows cap at a steady activating time, said seal member comes into contact with a seal portion which is provided in an inner portion of said housing so as to occlude said liquid chamber in the case that an operation of said device stops and the pressure within said pressure piping is lowered, wherein said seal member is obtained by attaching a flexible portion constructed by a rubber-like elastic body to an outer peripheral surface of a rigid plate, such that said rigid plate is set so that an outer diameter is smaller than an inner diameter of a flange portion provided in said seal holder, and said flexible portion is set so that an outer diameter is larger than the inner diameter of said flange portion, wherein said flexible portion allows relative movement of said bellows cap by shear deformation on the basis of engagement with said seal holder, when said bellows cap moves away from said seal portion while the seal member is in contact with said seal portion in the case that the liquid confined in said liquid chamber thermally expands in a state in which said liquid chamber is occluded. an accumulator housing which has a port hole connectable to a pressure piping of a device; a bellows and a bellows cap which is arranged in an inner portion of said housing so as to partition an internal space of said housing into a gas chamber to which a high-pressure gas is charged and a liquid chamber which is communicated with said port hole; and a seal member which is retained to the port hole side of said bellows cap via a seal holder, wherein said seal member moves together with said bellows cap at a steady activating time, said seal member comes into contact with a seal portion which is provided in an inner portion of said housing so as to occlude said liquid chamber in the case that an operation of said device stops and the pressure within said pressure piping is lowered, wherein said seal member is obtained by attaching a flexible portion constructed by a rubber-like elastic body to an outer peripheral surface of a rigid plate, such that said rigid plate is set so that an outer diameter is smaller than an inner diameter of a flange portion provided in said seal holder, and said flexible portion is set so that an outer diameter is larger than the inner diameter of said flange portion, wherein said flexible portion allows relative movement of said bellows cap by shear deformation on the basis of engagement with said seal holder, when said bellows cap moves away from said seal portion while the seal member is in contact with said seal portion in the case that the liquid confined in said liquid chamber thermally expands in a state in which said liquid chamber is occluded.