A work support comprising: a support rod inserted in an insertion hole provided through a leading end portion of a housing, the support rod being configured to be movable in the housing toward a leading end side and toward a base end side in an axial direction of the support rod; a collet fitted over an outer peripheral wall of the support rod; a piston inserted in the housing so as to be movable in the axial direction, the piston being configured to be actuated by compressed gas supplied to an actuation chamber to drive the support rod for locking via the collet; an output member inserted in a cylinder hole provided in a base end portion of the housing so as to be movable in the axial direction; a biasing means configured to bias the support rod and the output member so that the support rod and the output member recede from each other; an inlet chamber provided in the cylinder hole and on the base end side in the axial direction relative to the output member; an outlet chamber provided in the cylinder hole and on the leading end side in the axial direction relative to the output member; a switching means structured by a communication hole opening onto a peripheral wall surface of the cylinder hole and by an outer peripheral surface of the output member, the switching means being configured to switch, through movement of the outer peripheral surface of the output member across the communication hole, between a state in which the actuation chamber is communicatively connected to the inlet chamber via the communication hole and a state in which the actuation chamber is communicatively connected to the outlet chamber via the communication hole; and characterized by a discharge passage including an annular space, created between the insertion hole and the support rod, and a discharge port provided at the leading end portion of the housing, the discharge passage being configured so that compressed gas in the outlet chamber is discharged through the annular space to an outside of the housing from the discharge port, wherein the work support is designed so that an amount of gas pushed out of the outlet chamber by the output member when the output member moves from its base-end-side limit position to its leading-end-side limit position in the axial direction is larger than a capacity of an accommodation chamber created on the base end side relative to the support rod due to movement of the support rod from its base-end-side limit position to a leading-end-side position. a support rod inserted in an insertion hole provided through a leading end portion of a housing, the support rod being configured to be movable in the housing toward a leading end side and toward a base end side in an axial direction of the support rod; a collet fitted over an outer peripheral wall of the support rod; a piston inserted in the housing so as to be movable in the axial direction, the piston being configured to be actuated by compressed gas supplied to an actuation chamber to drive the support rod for locking via the collet; an output member inserted in a cylinder hole provided in a base end portion of the housing so as to be movable in the axial direction; a biasing means configured to bias the support rod and the output member so that the support rod and the output member recede from each other; an inlet chamber provided in the cylinder hole and on the base end side in the axial direction relative to the output member; an outlet chamber provided in the cylinder hole and on the leading end side in the axial direction relative to the output member; a switching means structured by a communication hole opening onto a peripheral wall surface of the cylinder hole and by an outer peripheral surface of the output member, the switching means being configured to switch, through movement of the outer peripheral surface of the output member across the communication hole, between a state in which the actuation chamber is communicatively connected to the inlet chamber via the communication hole and a state in which the actuation chamber is communicatively connected to the outlet chamber via the communication hole; and characterized by a discharge passage including an annular space, created between the insertion hole and the support rod, and a discharge port provided at the leading end portion of the housing, the discharge passage being configured so that compressed gas in the outlet chamber is discharged through the annular space to an outside of the housing from the discharge port, wherein the work support is designed so that an amount of gas pushed out of the outlet chamber by the output member when the output member moves from its base-end-side limit position to its leading-end-side limit position in the axial direction is larger than a capacity of an accommodation chamber created on the base end side relative to the support rod due to movement of the support rod from its base-end-side limit position to a leading-end-side position.