PURPOSE: To measure a point where a wire is broken, by measuring the electrostatic capacity of one end of each of two core wires of a cable and the electrostatic capacity of one end of the connected two core wires, and performing the computation using the measured values and the entire length of the known cable.

CONSTITUTION: A capacity measuring and operating device 5 measures the electrostatic capacities between a pair of core wires 2 and 3 and a shield conductor 4, and computes the distance to a point, where the wire is broken. The ends A and B of the core wires 2 and 3 are connected to the measuring device 5 through switches SA and SB and commonly connected by a switch SW. At the time of measuring, the other ends F of the core wires 2 and 3 are short-circuited. At first, SW and SB are turned OFF and SA is turned ON, and the electrostatic capacity CA is measured. Then, SB is turned ON and SW and SA are turned OFF, and CB is measured. Thereafter, SW is turned ON and either of SA or SB is turned ON, and CW is measured. The computation shown by an expression is performed based on the entire length of LT of a known cable, which is inputted separately by the operating device 5. Thus the distance LS to a point D, where the wire is broken, from the ends A and B can be computed.