PURPOSE: To carry out stable automatic feed water, forming high sensitive supplementary feed air system, by installing negative pressure detecting position of the pump in the automatic supplementary feed air device to the lowest pressure portion in the pump.

CONSTITUTION: An automatic supplementary feed air device 22 to supply air reduced in the pressure tank 14 is divided into two chambers 26, 28 with a membrane e.g. diaphragm 24 in the inner part. The position near point B which proceeds by nearly the length of 1/5 from the initial point A of eddy current water chamber 10 with a full length of L into the discharge side 12 is at the lowest pressure, it is the optimum position as the negative pressure detecting section 42 of the automatic supplementary feed air device 22. Then the chamber 26 of the device 22 is connected to the negative pressure detector 42 of suction side lower pressure section in the eddy current waterway 10, stable automatic ffed water can be carried out usually by permitting to supplementary feed air operation using strong vacuum pressure usually in spite of the low suction lift.