PURPOSE: To enable attainment of high sensitivity by executing a control so that a sensed-light signal level be always fixed.

CONSTITUTION: A light source 22 supplying light to an optical fiber sensor 21, a light source driving amplifier 23 having two inputs and driving this light source, a light-sensing unit 24 sensing a light signal from the optical fiber sensor 21 and outputting a sensed-light signal, and a sensed-light amplifier 25 amplifying the sensed-light signal, are provided. Moreover, a filter 27 which takes out a DC component from an output of this sensed-light amplifier 25 and outputs it to one input terminal 26 of the light source driving amplifier 23, a reference voltage generating element 29 which generates a driving reference voltage and outputs it to the other input terminal 28 of the light source driving amplifier 23, and an error convergence circuit 31 which feeds back an output 30 of the light source driving amplifier 23 to the input of this amplifier, are provided and thereby a construction is made so that the level of the sensed-light signal from the optical fiber sensor be always fixed.