PURPOSE: To detect frosting of an air conditioner correctly through detection of change in the natural frequency that is caused by frosting of an electrostrictive oscillator disposed in the vicinity of an evaporator.
CONSTITUTION: An electrostrictive oscillator 2 oscillated by an oscillation circuit 1 is fixed to evaporator 3, and oscillation frequency of the oscillation circuit 1 determined by the natural frequency of electrostrictive oscillator 2 is afforded to band- pass filter circuit 4. With such as arrangement, if an oscillation frequency indicating "frosting" is afforded to the circuit 4, it gives an output to detecting circuit 5, which in turn gives an output to switching circuit 6 for starting defrosting operation. Here, since oscillation frequencies produced by circuit 1 in the state that there is no frosting are higher than the band frequency of circuit 4, any output is given to detecting circuit 5. Thus, since, in accordance with the above arrangement, frosting of an evaporator is detected from the lowering of natural frequency of an electrostrictive oscillator, frosting can be detected highly accurately.