The light reception unit is disposed behind each of a plurality of light sources that emits signal lights having mutually different wavelengths in a forward direction and receives a backlight emitted backward from each of the light sources. The synchronizing signal generation unit generates a synchronizing signal serving as an electric signal synchronized in phase with a drive signal for driving each of the light sources. The multiplication unit multiplies the synchronizing signal and a light reception signal serving as an electric signal obtained by receiving the backlight by the light reception unit. The light power detection unit detects power of the signal light by multiplying an integrated value of a multiplication signal obtained by multiplying the synchronizing signal and the light reception signal by the multiplication unit and a power ratio of the signal light to the backlight stored in a predetermined storage unit.