PURPOSE: To enable proper prevention of the occurrence of cavitation phnomenon, by a method wherein, based on the temperature of fluid on the suction side, a reference suction flow rate responding to the number of revolutions is corrected, and by means of a pump trip set signal optimum to a fluid temperature, a suction flow rate is monitored.

CONSTITUTION: A number of revolutions signal S1 from a speed detector 6 is inputted to a pump trip flow rate setter 7. A suction temperature signal S5 from a temperature detector 11 situated in a suction line 3 is inputted to a temperature compensator 12. A before-compesation set signal S7 from the pump trip flow rate setter 7 and a compensation signal S6 are inputted to an adder 13, and the adding result is outputted as a pump trip flow rate set signal S3. A comparator 8 compares the signal S3 with a flow rate signal S2 from a flow rate detector 5, and when the signal S2 is lower than the S3, a pump trip signal S4 is outputted to an interlock circuit 9, alarm is sounded, and simultaneously pump trip is effected. This constitution enables proper prevention of the occurrence of cavitation phenomenon.