PURPOSE: To approximate the lift curve of a variable speed pump throughout a total delivery flow rate area through a similar curve formula and to compute and detect a delivery flow rate through a single computing method.

CONSTITUTION: A single similar curve formula approximating the lift curve of a variable speed pump 20 through secondary-fifth formula throughout a total delivery flow rate and a loss resistance curve formula of the variable seed pump 20 are stored by a computing means 14. The computing means 14 equalizes pumping head from the two formula to each other and leads out a formula indicating a delivery flow rate of an intersection between the two formula. The number N of revolutions from a number of revolutions detecting means 24 and water levels H1 and H2 from a suction water tank water level detecting means 12 and a delivery water tank water level detecting means 18 are inputted to the lead-out formula during operation of the variable speed pump 20, and a delivery flow rate Q of an intersection is computed through Newton.Lapson method.