PURPOSE: To detect the fact that tar is deposited on a pot of a pot-type petroleum combustor, so as to comply with the situation before the tar deposition becomes serious.

CONSTITUTION: A blower 3 and a fuel pump 6 are attached to a pot having the functions of a vaporization chamber of fuel and a combustion chamber, and the presence flame S formed in the pot is detected by the combination of a flame electrode 7 whose tip is extended into the pot with a flame detecting device 8. A first timer 9 is started in the operation of extinction when the fuel pump 6 is stopped, and an error circuit 10 is operated when the combustion flame can be detected for the set time or more, so as to inform the operator of the abnormality, moreover a second timer 11 is operated from the time when the combustion flame can not be detected, and the blower 3 in the post- purge operation is stopped in the count-up.