PURPOSE: To enable to obtain the flow amount of the variable speed pump by an operation only without measuring the actual flow amount by a method wherein simple operating units such as a subtractor, a multiplier, a divider and a function generator are combined.

CONSTITUTION: The signals of a suction well level meter 2 and a discharging well level meter 3 are inputted into the subtractor 5 in the flow amount operating device 4, and the actual lift HA of the pump may be obtained as the result of the operation. On the other hand, the signal of a tachometer 7 of the pump is inputted into the divider 8 to obtain the value of (100/N; N is revolving number) and, further, the signal is inputted into the function generator 9, having square-law characteristics, to operate the value of (100/N)2. The outputs of the function generator 9 and an actual lift operator 5 are outputted to the multiplier 10 and the lift HB of a similar point B upon N=100%, which is corresponding to the operating point A under the revolving number N and the actual lift HA may be obtained. This lift HB is inputted into the function generator 6 having the Q-H characteristics upon N=100% and whereby the flow amount QB may be obtained. Further, a revolving number N is multiplied in the multiplier 12 by a signal passed through the multiplier 11 multiplying 1/100 and whereby the actual flow amount QA may be operated.