PURPOSE: To enable only one micro-computer to be accommodated for a plurality of combustion devices by a method wherein each of input ports is provided with a frequency setting device capable of selecting more than two of each of a maximum and a minimum frequency by varying an input voltage.

CONSTITUTION: A micro-computer 1 is operated with 0V and 5V being applied as a power supply voltage. It is provided with an input port A2 for setting the maximum frequency, an input port A3 for setting the minimum frequency and an output part A4 for a pump 2. A frequency setting part 3 for sending a frequency setting signal to each of the input ports of the micro-computer 1 is comprised of resistors R1 to R4. Each of more than two frequencies of the maximum value and the minimum value can be selected by varying an input voltage for each of the input ports. With such an arrangement, it is possible to accommodate for a plurality of combustion devices without writing the upper or lower limit conditions again to the micro-computer 1.