To adjust an opening speed and a closing speed of an open valve separately not to cause a water-hammer action.

The open valve 1C of a type which opens a valve body 9 by pressuring a cylinder room 12 has piping 14 connected to the cylinder room and a flow control device 30 in the piping to adjust flow volume of water flowing in the piping. The flow control device adjusts to differentiate flow volume in the direction where water flows in the cylinder room from flow volume in the direction where water flows out of the cylinder room. The flow volume in the direction to flow in the cylinder room is adjusted to be less than that in the direction to flow out of the cylinder room, and not the closing speed of the valve body but the opening speed of the valve body is reduced. The flow control device comprises a valve member 31 with a structure of a check valve and an orifice member 33 to control a flow volume.