PURPOSE: To keep a air-fuel cost and keep a minimum current value regardless of kind of gas by a method wherein a driving circuit is provided with an F-i pattern modification circuit, a minimum current adding circuit and a proportional valve impressed voltage circuit for adding both circuits.

CONSTITUTION: At first, when a condition is set in such a way as a minimum combustion can be attained, data got from CPU becomes zero and then a voltage of 0V is outputted from an F-i pattern modification circuit A. In turn, a minimum current adding circuit B always outputs a voltage corresponding to MIN current and MIN current is given to a gas proportional valve D under the minimum combustion time. The proportional valve D is adjusted for every kind of gas in such a way as a secondary specified voltage at the secondary side becomes a minimum gas pressure with this current. Then, the value is set in such a way as it shows the maximum combustion and adjusted in such a manner as the secondary measured pressure in the proportional valve D becomes the maximum gas pressure. With this arrangement, an amount of air and an amount of gas are accurately matched to each other and the minimum current can be assured regardless of the kind of gas.