PURPOSE: To improve precision and reliability of detection by maintaining always set oxygen concn. as criterion for oxygen deficiency with regard to optional amt. of combustion, namely, to optional power generation.

CONSTITUTION: An ignition current value is detected by an ignition current detecting circuit 6. The ignition current value inputted to an ignition current signal comparing circuit 8 as stable ignition current output by an ignition current output circuit 7 is held always at a higher value than a criterion current signal inputted to the ignition current signal comparing circuit 8 after being converted from a combustion amt. controlling output depending on an oxygen concn. (e.g. 18%) set as a limit of oxygen deficiency condition for a criterion signal converting circuit 5. As oxygen concn. of the air for combustion decreases, the output value of the ignition current approaches the criterion current signal value. When the oxygen concn. decreases beyond 18%, the ignition current value becomes smaller than the criterion current signal value. The ignition current signal comparing circuit 8 generates in this state an alarm signal finally, and an alarm output circuit 9 operates to actuate an alarm lamp or buzzer, etc. and performs combustion interrupting operation successively or simultaneously, and automatically.