To prevent primary side pressure from increasing from a set value due to generation of water hammer and to prevent generation of a valve closing defective by providing a pressure relief valve on a primary side of a water level differential ball tap and performing a pressure adjustment when primary side pressure is increased from the set value.

At the time of a full water level, a float 46 is located at an ascending location, a cylinder 20 is sealed by a seal member 26 of a piston 24 and a water discharge valve 22 is in a closing state. When water hammer is generated in a primary side channel 14 and primary side pressure is increased from a set value in this state, increased pressure is discharged from a pressure relief valve 2 to the outside. Next, when a water level is lowered, an operating rod 42 is oscillated downward by lowering of the float 46 via a lever rod 44 and a link 45 and the water discharge valve 22 is opened. When seal of the seal member 26 for the cylinder 20 is released while this valve 20 is opened, the primary side channel 14 and a pressure chamber 27 are communicated and the water discharge valve 22 is fully opened at once.