PURPOSE: To enable a water hammer phenomenon in opening an open valve to be surely prevented and the pressure resisting grade of apparatus members in the secondary side piping system to be reduced at ease by operating a valve body of a pressure regulator according to liquid pressure in a second secondary chamber with a liquid pressure changeover device in the beginning of opening the open valve.

CONSTITUTION: When a fire takes place to operate a fire detector 1, a receiving board 3 operates a motor section 41 according to a fire signal to open a starting valve 24. Thus, liquid pressure in a cylinder chamber 20 of an open valve 6 is exhaust through an operation chamber 17 from a drain pipe 25 and reduced. The open valve 6 is opened by the liquid pressure in a primary chamber 8, and fire extinguishing liquid from the primary chamber 8 flows into an inflow detecting chamber 22 through a peripheral wall 33 mainly with high pressure by a pressurizing means like a pressurized air tank and from a secondary pipe 7 into the secondary side piping 5 and piping 31. The liquid pressure in the inflow detecting chamber 22 is increased to operate a pressure switch 39 and inform the receiving board 3 of opening the open valve 6.