PURPOSE: To detect easily and exactly a mismatching attenuation quantity of an attenuation quantity, and existence of discontinuity of the attenuation quantity by a layout of a simple measuring instrument, by inputting a voltage of an output terminal of a variable resistance attenuator to a horizontal axis or a vertical axis of an oscilloscope, and inspecting an electric characteristic by an action of a bright line generated on a cathode ray tube of the oscilloscope.

CONSTITUTION: When a continuous variable resistance attenuator is inserted into a v ariable resistance attenuator 4, and an attenuation quantity of the attenuator is set to the maximum value, output voltage Vb becomes small, and a luminescent point is positioned at a point A. Also, when a set position of the attenuator is shifted in the direction of the minimum attenuation quantity, the luminescent point moves in the right direction, stops at a position B point of the minimum attenuation quantity, a bright line can be drawn on a cathode ray tube, and mismatching attenuation quantity and the maximum and minimum attenuation quantities can be measured easily and quickly. Also, a response speed of an oscilloscope 7 is remarkably higher than that of a conventional level meter, etc., therefore, in case when discontinuity and a backward movement of an attenuation quantity variation characteristic exist, they are displayed on the cathode ray tube, and can be detected exactly.