PURPOSE: To simply diagnose the deterioration state of an electric wire/cable by bonding a strain gauge to the constitutional insulating material of the electric wire/cable and monitoring the volumetric change of the insulating material accompanying by deterioration.

CONSTITUTION: Strain gauges 5 detecting minute strain are bonded to the outermost layer of a three-core low voltage cable 1 constituted by receiving conductors 3 each coated with an insulating material 2 such as crosslinked polyethylene in a polyvinyl chloride sheath 4 being an insulator at several appropriate places along the cable 1 before laying and operation or after laying by an adhesive. The volume of the cable 1 is changed by deterioration after the elapse of a long time to appear as strain. This strain is detected by the strain gauges 5 and the electric signals thereof are inputted to a measuring condition 7 through lead wires 6 to be monitored. By this constitution, the deterioration of the cable 1 and the degree thereof can be always detected in a live wire state.