AUTOMATIC ANALYZER

Disclosed is an automatic analyzer which can detect a trace amount of reaction solution at a high S/N with stability. The analyzer allows a photomultiplier (111) to detect light from the reaction solution (109) containing a luminescent substance (114) through an optical window (102). To process the output from the photomultiplier to analyze the amount of the luminescent substance (114) contained in the reaction solution (109), an optical transmission system (110) is interposed between the optical window (102) and the photomultiplier (111). The optical transmission system includes a light inlet opposed to the optical window; a light outlet opposed to the light-receiving surface of the detector; and a reflector on which an incident beam of light from the light inlet is reflected to propagate to the light outlet. This configuration allows the effects of temperature-dependent noise from a flow cell (101) to be reduced while preventing a drop in the amount of light from the luminescent substance (114), thereby implementing analysis at a high S/N.