PURPOSE: To shorten a test time and to prevent malfunction from occurring by providing an oscillator, a rectifying and smoothing circuit, an A/D converting circuit, a control and arithmetic means, etc.

CONSTITUTION: The variable frequency type oscillator 1 sends its output to an electronic device 2 to be inspected. The rectifying and smoothing circuit 7 rectifies the output of the electronic device 2 on full-wave basis to generate a DC level and the A/D converting circuit 5 converts the output of the rectifying and smoothing circuit from analog to digital. Then, the control and arithmetic means 6 sets the oscillation frequency of the oscillator 1 and also sets a sampling period corresponding to the set oscillation frequency in a sampling circuit. Namely, the sampling period is shorter and shorter and the frequency of sampling is larger and larger as the set oscillation frequency is lower and lower. For the purpose, all sampled values are averaged and the mean value is equal to the value when a capacitor is switched. Namely, electrostatic capacity need not be increased, so the time up to the stabilization of the output level of the rectifying and smoothing circuit is shortened in the beginning of the oscillation of the oscillator.