A photodetector detects fluorescence emitted from a sample placed on a substrate of a DNA chip. There is an irradiation optical system for guiding irradiation light by a first optical waveguide, gathering the irradiation light by a first lens and irradiating the sample. A reception optical system gathers fluorescence at an input-side end surface of a second optical waveguide by a second lens and guides the fluorescence to a measuring unit. The irradiation optical system and the reception optical system are separate light guiding paths. The reception optical system is of a confocal optical system in which a focal point on the sample is identical to a focal point at the input-side end surface of the second optical waveguide of the reception optical system.