PURPOSE: To search a trouble position accurately by making a constant current flowing intermittently in a system of searching a short-circuiting trouble position in which a constant current flows in at one end of a submarine cable to measure a voltage drop at one end of the submarine cable.

CONSTITUTION: A 10Hz oscillator OSC applis a pulse into a decimal ring counter COUNTER at an interval of 100mS. When counts of the decimal ring counter COUNTER are 0W4, an OR gate G outputs an output signal to drive a driving circuit DRIVE and operates a relay RL. When counts of the decimal ring counter COUNTER are 5W9, the OR gate G fails to produce no output signal and the relay RL fails to operate. Thus, only at 500mS, the relay RL is operated and such a driving of the relay activates a relay contact rl. When the ratio between the energization time and the power failure time is set at 1:1 or 1:3 approximately and the energization time is 500mS, DC current flows intermittently and a voltage drop is measured accurately without raising the temperature of a Zener diode thereby enabling the detection of trouble points.