PURPOSE: To prevent the occurrence of gas lock when bubbles produced from bubble generating liquid are accumulated in a pump, by opening an intake side valve body having buoyance in the liquid to discharge the bubbles out of the pump.

CONSTITUTION: Bubbles produced from liquid in a bore 2 of a casing ascend in the liquid to form a bubble layer on the lower surface of a valve body 11. The valve body 11 loses buoyancy and somewhat descends to open a valve so that the bubble layer ascends in the liquid through the opened valve and passes through a intake side check valve 3 and a gas vent pipe to be discharged into the atmosphere. Since the bubble discharging action is carried out normally automatically, the bubbles are not accumulated in the bore 2 of the casing and a valve chamber 9 so that gas lock do not take place and normal intake and discharge are continued.