PURPOSE: To make it possible to emphasize the change of a discontinuous part, by employing a DC voltage generating source, to which a minute AC signal is superposed, as the bias voltage source of a gun-diode while providing a detector circuit of a microwave, which is modulated by said AC signal, from an oscillation apparatus.

CONSTITUTION: A microwave generated from a gun-diode oscillation apparatus 1 is biased by a signal obtained by superposing an AC signal to a DC power source to be converted to a modulated signal. This signal is detected by a detector 6 and enters a low frequency amplifier 8 while the high frequency component thereof is removed by a condenser 7. The output of said amplifier 8 is converted to a DC signal by a smoothing circuit 9 to be inputted to an X-Y recorder 10. This converted signal shows the variation in a microwave signal and is minute oscillation output to applied voltage. In this case, the applied voltage of the gun diode is inputted to the other input terminal of the recorder 10 through an inductor 11 to observe the minute oscillation output to the applied voltage. When a discontinuous part is present in oscillation characteristics, the change thereof appears in an emphasized state and observation becomes easy.