PURPOSE: To make full use of a variable speed pump as well as to improve a flow control function, by calculating a cut-off operating point in the variable speed pump on the basis of an output signal out of a conveying pressure gauge measuring hydraulic pressure at the conveying side and another output signal out of a conveying flow meter.

CONSTITUTION: In case of a system controlling a conveying flow rate with a variable speed pump 1 and plural units of fixed speed pumps 2, both the existing conveying pressure value and conveying flow value are taken into an arithmetic unit 8 by means of a conveying pressure gauge 7 and a conveying flow meter 3. At this arithmetic unit 8, a cut-off operating point in the variable speed pump is found and the result is outputted to a flow adjustment and pump number control device 4. As a result, for a while reaching to the cut-off operating point, a variable speed signal is outputted to a speed controller 6 of the variable speed pump, while at the cut-off operating point, a fixed pump decreasing command is outputted so that a flow control function is thus improved.