A fourth port P4 of a first WDM-type optical coupler 18 and a fifth port P5 of a second WDM-type optical coupler 19 are optically connected, and a second port P2 of the first WDM-type optical coupler 18 and an eighth port P8 of the second WDM-type optical coupler 19 are optically connected. A light source 11 for emitting excitation light is connected to a first port P1 of the first WDM-type optical coupler 18, and a light-receiving section (light detector) 15 is connected to a sixth port P6 of the second WDM-type optical coupler 19. Excitation light is emitted through a seventh port P7 of the second WDM-type optical coupler 19 toward an observed subject, fluorescence from the observed subject is acquired through the seventh port P7 of the second WDM-type optical coupler 19, and input fluorescence is detected by the light-receiving section 15.
