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