PURPOSE: To prevent of a burning motor due to idle running by detecting an output of solar battery as compared with an operating frequency of variable frequency inverter being less than a given value, so as to stop an operation of the variable frequency inverter.

CONSTITUTION: A solar pump system is composed of a solar battery 1, a reverse-current blocking diode 2, a capacitor 3, a variable frequency inverter 4, a pump motor 5 consisting of an AC motor and a pump 6 for said system. Also, a control unit for said solar pump system is composed of a resistance type potential divider 7 for solar battery voltage, a voltage setter 8, a voltage controller 9, a V/F converter 10, a PWM circuit 11, an output voltage instructor 12, an output current sensor 13 and a decision means 17. Said decision means 17 is composed of a function generator 14 outputting a predetermined value for a frequency signal and a comparison and decision device 15 comparing an output current of the solar battery 1 with an output of the function generator 14 for instructing a drive or stop of the inverter, so as to prevent the pump motor 5 from running at an excessive speed.