PURPOSE: To permit two kinds of gas fuels to be combusted continuously, by supplying a high calorific fuel when a quantity of flow of a low calorific fuel has reached a value greater than a prescribed value.

CONSTITUTION: A flow meter 8 and a flow control valve 9 are provided with a low calorific fuel supply system. When only a single fuel is combusted, the quantity of flow of a low calorific fuel is measured by the flow meter 8 and when it has become greater than a prescribed value, the flow control valve 9 controls to supply a prescribed quantity of flow. The shortage of fuel is covered by supplying a high calorific fuel for operation by mixed fuel combustion. With this arrangement, two different kinds of gas fuels can be continuously combusted.