PURPOSE: To replace singly a photodetector by connecting a plate-like member stuck to an optical transfer means and a photodetecting surface of the photodetector by packing and sealing up a transparent liquid whose refractive index is nearly equal to that of both of them, in its gap.

CONSTITUTION: A plate-like member 17 stuck to a light emitting surface 12b of an optical transfer means 12 is constituted of an optical filter for making only blue accelerated luminous light transmit and shielding red excitation light, and has a refractive index which is roughly equal to that of the optical transfer means. A photodetecting surface 13a of a photodetector is placed at a position opposed to the plate-like member 17, and this gap is packed with a silicon oil 18. The photodetecting surface 13a and the silicon oil 18 have a refractive index which is roughly equal to that of the plate-like member 17, therefore, the accelerated luminous light transferred from the optical transfer means 12 does not generate an interfacial reflection but is made incident into the photodetector 13. In this way, since said member is connected through a liquid, only the photodetector can be replaced singly and easily, and also no interfacial reflection is generated on the light emitting surface and the photodetecting surface of the photodetector.