PURPOSE: To detect one operation condition of a plurality of sensor sections of an optical fiber, by varying and outputting an intensity modulation light having a specified frequency from a light emitting section with the action of a sensor section having a delay optical path to detect a specified frequency component alone with a light receiving section.

CONSTITUTION: An oscillator signal is modulated by frequency f1 and outputted to an optical fiber 2 and an intensity modulation light overlapped with the frequency f1 is taken out to the fiber 2. An intensity modulation light passing through a sensor section 81 is turned to P1 while that passing through an optical fiber 111 to P2 by way of an optical branching device 91. Here, for example, when the sensor section 81 fails to operate, the light P1 incident is outputted as intact while the light P2 is delayed by π/2 in phase, which causes the frequency f1 to be cancelled in the intensity modulation light P0 added with an optical multiplexer 101. On the other hand, when the sensor section 81 operates, the light P1 fails while the light P2 is outputted, eventually making the light P0 equal the light P2. An AC signal with the frequency f1 is detected with a bypass filter within a light receiving section 3 thereby clearly indicating that the inspecting section 81 is operating.