PURPOSE: To prevent the occurrence of water hammer by converting the horizontal turning of a handwheel shaft into a component force by which a valve spindle is horizontally turned via a guide member and a guide piece.

CONSTITUTION: When a handwheel shaft 18 is turned, a guide member 25 rises, and the round stem part 27b of a guide piece 27 in a guide groove 28 pushes a slant groove part to cause a component force by which a valve spindle 13 is horizontally turned. Since the valve spindle 13 is horizontally turned with a slow speed as compared with the turning of the handwheel shaft 18, a valve body 9 is opened with a slow speed. When a handwheel 17 is further turned, the round stem part 27b comes to take the position in the groove at the upper end, and the valve spindle 13 is turned approximately 90° so that the valve body 9 becomes in an open condition. Thus, the opening or closing of the valve body 9 is carried out gradually, so that the occurrence of water hammer can be prevented.