PURPOSE: To perform speed regulations between a forerunning machine and an incoming one in a short time, by calculating the desired speed regulating revolution on the basis of a lift-revolution characteristic in a variable speed pump and thereby decelerating the forerunning machine but accelerating the incoming one with the said revolution.

CONSTITUTION: In an operation circuit 9 of a control unit 8, those of a discharge-lift characteristic at every parallel operating unit number, a lift-revolution characteristic at every parallel operating unit number and a lift characteristic in time of cut-off operation on each individual pump are all stored in memory. When an accelerating command is issued during automatic controlling, both suction and discharge pressures are detected by pressure gauges 6 and 7, then the existing operating unit number is calculated. Next, the existing total lift is calculated whereby a speed regulating revolution in case of the pump unit number being changed is outputted as the desired value on the basis of the characteristic curve stored in the operation curcuit 9. With this constitution, speed control takes place in a way of deceleration a forerunning machine but decelerating an incoming one with the said desired value.