To supply a constant power all the time to a heater in spite of the reduction of a battery voltage by increasing a quantity of power application to the heater by extending an ON duty period when the voltage of the battery for use in a driving power supply of the heater.

This device, in which a heat generation from the heater H driven by the constant power using a pulse voltage is transferred to a temperature sensor using a fluid as a medium and a flow rate of the liquid is measured based on the output of the temperature sensor, comprises voltage monitoring means Q1 for monitoring the voltage of the battery for use in the driving power of the heater H and ON duty control means R15, R16, Q1, Q2, C1, and OP1 for extending the ON duty period for the pulse voltage drive of the heater H when the voltage reduction is detected by the voltage monitoring means Q1.