PURPOSE: To avoid disable measuring section near an incident end and short detection distance for a broken point, by preventing the reflection of the light from the incident point of an optical fiber through the insertion of an optical delay element having a slightly shorter delay time than a half the operating time of a photo switch.

CONSTITUTION: Light emission from a light emitting element 1 is made incident to an optical fiber 5 to be measured via an optical delay element 3, etc. inserted between an optical switch 2 and an optical connector 4 with a slightly shorter delay time than a half the switching operation time of the switch 2 to a photodetector 6. No refleting light from the connector 4 is made incident to the detector 6 with the element 3 and the saturation of the element 6 can be prevented. The reflected light from a broken point near the incident end of the fiber 5 is received at the detector 6, avoiding disable measurement near the incident end. Further, the element 3 has no attenuation and the detection distance of the broken point does not become short.