To provide a valve opening/closing machine preventing water hammer by automatically delaying a valve closing speed of a valve element in a small opening region than a valve closing speed in a region except for the small opening region.

This valve opening/closing machine is provided with an input shaft 50, an output shaft 51 connected to a valve stem 3, and a power transmission means 7 interposed between the input shaft 50 and the output shaft 51. The power transmission means 7 is provided with a worm 70 provided in the input shaft 50, a Geneva worm wheel 71 provided in the output shaft 51 and geared with the worm 70, a worm wheel 73 provided in an intermediate shaft 72 and geared with the worm 70, an intermediate small gear 74 provided in the intermediate shaft 72, and a Geneva large gear 75 provided in the output shaft 51 to be geared with the intermediate small gear 74. The valve closing speed in the small opening region of the valve element 2 is automatically delayed than the valve closing speed in the region except for that.