Optical radar principle of pulse reflection is applied, a GaAs single heterojunction laser is adopted as its pulse light source and an aspherical lens system is used to couple the light into the fiber. The reflected pulses are detected by a PIN diode, amplified and sent to a special counter. The displayed digits shows the distance between the fault in the fiber and its measured end. Accuracy of the apparatus is 2.5M; measuring length: 1.5kM (attenuation of fiber: 10dB /kM).