To attain alarm detection without being affected by temperature fluctuation.

An LD module 5 of a PD incorporating type consisting of LD 3 and PD 4 converts an electric signal from an input terminal 2 to an optical signal and an amplifier 7 respectively outputs a positive phase voltage signal 102 to a peak detection circuit 10 and reverse phase voltage signal 103 to a peak detection circuit 13. Voltages not affected by temperature are obtained at both ends of a resistance element 14 being the outputs of the respective peak detection circuits 10 and 13, and the voltages of both ends of this resistance element 14 is converted to unipolar outputs by a differential amplifier circuit 21 at a next level and then outputted to a comparator 22. Then the comparator 22 compares this unipolar output with a threshold value voltage supplied by an input terminal 23 and performs alarm detection.