An instrument for measuring a biogenic substance, using the confocal optical system that is simplified and reducing measurement error caused by the difference in physical conditions or temperatures due to weather of the season is realized. The instrument for measuring a biogenic substance, using the confocal optical system comprises the confocal optical system comprised of a laser source (22), a lens system (23) for collimating light from the laser source (22), a half mirror (24) for causing the light collimated by the lens system (23) to be transmitted therethrough, a lens system (25) for converging the light that is transmitted through the half mirror, a lens system (26), a pinhole (27) and a photodetector (28), and a body for housing these elements and provided with a run button and an exit aperture (44a).
