To discriminate whether a fuel gas flowmeter and an air flowmeter are abnormal or not with a simple constitution without increasing costs.

At least one of a fuel supply pathway 3 and an air supply pathway 7 is provided with pressure sensors 14, 16 for measuring differential pressure between an upstream side and a downstream side of orifices 13, 15, and an abnormality diagnosing means 17 is disposed to discriminate whether the flowmeter disposed at the side of the supply pathway provided with the pressure sensors 14, 16 is abnormal or not by comparing a flow rate corresponding to the measured differential pressure of the pressure sensors 14, 16 and the measured flow rate of the fuel gas flowmeter 4 or the air flowmeter 8 disposed at the side of the supply pathway provided with the pressure sensors 14, 16, at the side provided with the pressure sensors 14, 16 of the fuel supply pathway 3 or the air supply pathway 7, when the output of a combustion burner 2 is the predetermined output for diagnosing abnormality.