PURPOSE: To secure to detect usual plugging of filter and discover trouble on execution of works by providing a temperature detector to tell a temperature adjacent to a combustion chamber, an indicating means to indicate a trial run, and a comparing and discriminating part which tells a temperature detected is going to exceed the basic temperature at operation or the one at the trial turn when indicated it.

CONSTITUTION: In the case of trial, the basic temperature of a comparing and discriminating part 27 is set at a degree equivalent to that, for instance, just before a filter is plugged completely, a gas burner 2 and motor 9 are driven, and temperature detected by a temperature detector 16 is read and compared at the comparing and discriminating part 27. In operation, the basic temperature of the comparing and discriminating part 27 is set at a safety degree that does not reach overheat, the gas burner 2 and motor 9 are driven, and temperature detected by the temperature detector 16 is read and compared at the comparing and discriminating part 27. In trial, when the comparing and discriminating part 27 outputs, the gas burner 2 and motor 9 are stopped, and something unusual is indicated by an indicator 21, while a buzzer 22 is sounded to inform it for a given time.