When constantly detecting the opening of the flowrate adjusting valve (3) that is driven by a stepping motor (34) with the use of a rotary encoder, etc., the precision of the opening control is increased, but the control device becomes complicated. Therefore, although the opening of the flowrate adjusting valve can be controlled by means of the pulse number supplied to the stepping motor (34), it is necessary to correct positively the pulse deviation due to the step-out. In accordance with this invention, when the micro switch (5) positioned at a position that is adjacent to a completely turn off position and to which the gas amount required for igniting can be supplied is ON, the deviation of the pulse number and the actual opening is corrected.