PURPOSE: To detect a poor air supply and air discharge with simple configuration without installing air pressure switch and to reduce the number of component parts.

CONSTITUTION: An input voltage IX of a fan motor 9 during a pre-purging is detected and in the case that the input current IX is less than the first predetermined value I1, combustion is stopped in order to prevent any incomplete combustion under a judgement that the high level poor air supplying and discharging occurs. In the case that the input current IX is larger than the first predetermined value I1 and less the second predetermined value I2, it is judged as a low level poor air supplying or discharging state, an alarm lamp 32 is lit and a replacing of an air supplying filter is promoted. In addition, a poor degree of air supplying or discharging operation is displayed in response to the input current IX, and it can be visually acknowledged.