To assure detection of fluorescence produced from continuously supplied fine particles such as DNA bases with accuracy and excellent efficiency, fine particles supplied from a flow cell are introduced into a through-hole of a hollow photomultiplier. An optical fiber is disposed in axial alignment with the through-hole of the photomultiplier and irradiates an exciting light beam onto the fine particles staying in the through-hole. Light-induced fluorescence is thus detected by the photomultiplier which has a photocathode on an inner circumference of the through-hole.