PURPOSE: To obtain highly reliable test results to match actual use, by using an AC voltage as such to be applied to the core.

CONSTITUTION: An AC voltage is used as such as to be applied to a sample cable 1. When a deterioration test is carried out by the measuring circuit of this invension, a switch S1 is connected to the terminal (a) position to apply the AC voltage between the terminal and a shield layer 1b grounded to the core 1a of the cable 1. Subsequently, the switch S1 is connected to the terminal (b) position while the core 1a is grounded initially through a resistance R and then, after the switch S2 is closed, it is grounded through the resistance R. After a specified time, the switch S1 is connected to the terminal (c) position to release the grounding of the core 1a while the residual voltage of the cable 1 is measured with a voltometer 2. The deterioration in the cable 1 is decided from the residual voltage.