To relieve a water hammer phenomenon by stably controlling a flow rate.

An expansion unit EU having an operation chamber 33 communicating with a back pressure chamber 27 of a main valve 20, is interposed between a valve casing 16 having the main valve 20 for opening-closing a port 19 and a solenoid unit 32 incorporated with a plunger 31. The expansion unit EU is provided with an auxiliary valve 37. Both the auxiliary valve 37 and the main valve 20 are formed with pressure reducing passages 28 and 39 and pressure supply passages 30 and 42. When the pressure reducing passages 28 and 39 of both valves 20 and 37 are closed by the plunger 31 when closing a valve, among the operation chamber 33, a second chamber 33B increases in pressure in the first place, and then, pressure in a first chamber 33A increases via a sub-pressure supply passage 42. Thus, since the whole operation chamber 33 slowly increases in the pressure, a valve closing speed of the main valve 20 is also slowly performed.