An apparatus includes a system for guiding chemiluminescence and a system for preventing a variation in dark currents. The apparatus includes a first light shielding BOX having a sample container holder and a shutter unit therein, the shutter unit including a top plate which is partly formed by a movement of a plate member and a second light shielding BOX having a photodetector therein. While a measurement is not implemented, the shutter unit is closed to block entrance of stray light to the photodetector, and while a measurement is implemented, the plate member is moved to open the shutter unit and the tip of the photodetector is inserted into a through hole formed in the top plate, so that the distance between the bottom of the sample container and a sensitive area of the photodetector is reduced to several millimeters or less.