A work support comprising: a housing; a support rod supported by the housing so as to be movable in an up-down direction and configured to be advanced/retracted relative to a workpiece; an advance piston including a piston portion hermetically inserted in a tubular hole in a lower portion of the housing so as to be movable in the up-down direction; an advance-use pressure chamber provided below the piston portion; a biasing means configured to bias the support rod upward; and a retracting means configured to move the support rod downward via the advance piston, the work support further comprising: an ascent-detecting air supply port, a descent-detecting air supply port, and an air discharge port, which are provided to the housing; an air outlet provided at a leading end portion of the support rod, the air outlet being configured to be closed by the workpiece; an ascent-detecting air passage provided in the housing and in the support rod, the ascent-detecting air passage communicatively connecting the ascent-detecting air supply port and the air outlet to each other; a descent-detecting air passage provided in the housing, the descent-detecting air passage communicatively connecting the descent-detecting air supply port and the air discharge port to each other; and a valve mechanism configured to close the descent-detecting air passage when the advance piston is retracted to a lowered position by the retracting means, the valve mechanism including a valve surface provided on the piston portion and a valve seat provided on an inner surface of the housing. a housing; a support rod supported by the housing so as to be movable in an up-down direction and configured to be advanced/retracted relative to a workpiece; an advance piston including a piston portion hermetically inserted in a tubular hole in a lower portion of the housing so as to be movable in the up-down direction; an advance-use pressure chamber provided below the piston portion; a biasing means configured to bias the support rod upward; and a retracting means configured to move the support rod downward via the advance piston, the work support further comprising: an ascent-detecting air supply port, a descent-detecting air supply port, and an air discharge port, which are provided to the housing; an air outlet provided at a leading end portion of the support rod, the air outlet being configured to be closed by the workpiece; an ascent-detecting air passage provided in the housing and in the support rod, the ascent-detecting air passage communicatively connecting the ascent-detecting air supply port and the air outlet to each other; a descent-detecting air passage provided in the housing, the descent-detecting air passage communicatively connecting the descent-detecting air supply port and the air discharge port to each other; and a valve mechanism configured to close the descent-detecting air passage when the advance piston is retracted to a lowered position by the retracting means, the valve mechanism including a valve surface provided on the piston portion and a valve seat provided on an inner surface of the housing. an ascent-detecting air supply port, a descent-detecting air supply port, and an air discharge port, which are provided to the housing; an air outlet provided at a leading end portion of the support rod, the air outlet being configured to be closed by the workpiece; an ascent-detecting air passage provided in the housing and in the support rod, the ascent-detecting air passage communicatively connecting the ascent-detecting air supply port and the air outlet to each other; a descent-detecting air passage provided in the housing, the descent-detecting air passage communicatively connecting the descent-detecting air supply port and the air discharge port to each other; and a valve mechanism configured to close the descent-detecting air passage when the advance piston is retracted to a lowered position by the retracting means, the valve mechanism including a valve surface provided on the piston portion and a valve seat provided on an inner surface of the housing.