PURPOSE: To detect the abnormality in closing funtion of a gas control valve caused by mechanical trouble, by providing a pressure sensor to detect the internal pressure in a burner to a gas burner.

CONSTITUTION: A controller 7 receives a signal from a pressure sensor 8 which senses the internal pressure in a burner 5, as well as to outputs driving signals to control valves 2 and 3. The difference in the internal pressure in a burner during a combustion and non-combustion can clearly be discriminated by P21> P22>0 (mmHg). At the time of initial checking before combustion is started, gas control valves V2 and V3 are alternately driven within a specified time (t), and whether the internal pressure in a burner in each state is P22 or not is detected. If an output from the pressure sensor is P22 in a term t3Wt4 when the valve 2 is opened and the valve 3 is closed, it indicates that the closing function of a valve V3 is damaged. While, if the pressure is P22 within the term t4Wt5, it indicates that the closing function of a valve V2 is damaged.