PURPOSE: To disregard the change with the lapse of time in the output voltage of an oscillation circuit and to detect a change in an oscillation frequency due to the deterioration in components of an oscillation circuit.

CONSTITUTION: The circuit consists of an oscillation circuit 1 oscillating a prescribed frequency signal, a buffer circuit 2 extracting an oscillation signal from the oscillation circuit 1 to its external device, a high pass filter 3 cutting off the DC component of an output signal outputted from the buffer circuit 2, a waveform shaping circuit 4 waveform-shaping the output signal of the high pass filter 3, a low pass filter 5 converting the output signal of the waveform shape circuit 4 into a DC component and a voltage comparator circuit 6 discriminating whether or not the oscillation circuit 1 is oscillated from the DC component outputted from the low pass filter 5.