PURPOSE: To prevent occurrence of air-to-fuel ratio discrepancy which ac companies a change in the combustion air amount during fuel change-over and so on by changing pulse to a fuel supply device according to the air amount that is detected by providing a pulse generating section in a fuel control section.

CONSTITUTION: For instance, when the amount of fuel is changed over, signals inputted to AN1, AN2, and AN3 of a microcomputer 12 which are connected to a room temperature sensor 8, burner temperature sensing element 15, and room temperature setting volume 11' are compared at a comparison section 27, and output a specified signal, strong, weak, or others. Based on this signal a burner motor control section 30 supplies signal to an output terminal R3 to change-over the level of a burner motor 3 for combustion air supply to strong, weak, or others. On the other hand rotation signal of a windmill 102 of an air volume detector 100 that is inputted from an input terminal I2 is sent to a rotational speed detection section 31, and, furthermore, at an arithmetic section 32 output frequency is determined. A determined output frequency outputs pulse signal to an output terminal R5 from a pulse generating section 33 and drives a pulse pump driving circuit 4' through a hot coupler 20 and operates a pulse pump 4 for fuel supply.