A clamp device comprising: an engagement member, inserted in a hole in an object to be fixed, which is capable of gripping an inner circumferential surface of the hole, the engagement member being constituted by a plurality of engagement members circumferentially provided at predetermined intervals or a monolithic engagement member circumferentially provided in an annular shape; a clamping rod having a wedge part that is engaged on an inner circumference of the engagement member from a top end; a piston that causes the clamping rod to move axially, the piston having a piston rod protruding toward the top end so as to be coupled to the clamping rod; and a housing in which the engagement member, the clamping rod, and the piston are provided, a valve mechanism provided in the housing so as to be actuated by the piston rod or the clamping rod, the valve mechanism being opened or closed when the piston has moved to a last-stage stroke region in a lock direction of a total stroke of the piston, the clamp device being configured so that pressurized air is able to be supplied to an inlet of the valve mechanism, an annular space being formed between a cylinder hole in the housing and the piston rod, a valve member of the valve mechanism being formed in a cylindrical shape, the clamping device being characterised by the cylindrical valve member being hermetically inserted in the annular space so as to be movable axially, a valve face of the valve member being configured such that an annular end face provided on a base end of a step part of the cylinder hole is contactable for closure by the valve face from the base end, and when the piston has moved to the last-stage stroke region in the lock direction, the valve face being separated by the piston rod or the clamping rod from the annular end face toward the base end to form a gap through which the pressurized air supplied to the inlet of the valve mechanism is discharged toward an outlet. an engagement member, inserted in a hole in an object to be fixed, which is capable of gripping an inner circumferential surface of the hole, the engagement member being constituted by a plurality of engagement members circumferentially provided at predetermined intervals or a monolithic engagement member circumferentially provided in an annular shape; a clamping rod having a wedge part that is engaged on an inner circumference of the engagement member from a top end; a piston that causes the clamping rod to move axially, the piston having a piston rod protruding toward the top end so as to be coupled to the clamping rod; and a housing in which the engagement member, the clamping rod, and the piston are provided, a valve mechanism provided in the housing so as to be actuated by the piston rod or the clamping rod, the valve mechanism being opened or closed when the piston has moved to a last-stage stroke region in a lock direction of a total stroke of the piston, the clamp device being configured so that pressurized air is able to be supplied to an inlet of the valve mechanism, an annular space being formed between a cylinder hole in the housing and the piston rod, a valve member of the valve mechanism being formed in a cylindrical shape, the clamping device being characterised by the cylindrical valve member being hermetically inserted in the annular space so as to be movable axially, a valve face of the valve member being configured such that an annular end face provided on a base end of a step part of the cylinder hole is contactable for closure by the valve face from the base end, and when the piston has moved to the last-stage stroke region in the lock direction, the valve face being separated by the piston rod or the clamping rod from the annular end face toward the base end to form a gap through which the pressurized air supplied to the inlet of the valve mechanism is discharged toward an outlet.