PURPOSE: To determine the reaction rate of a chemical reaction such as enzymatic reaction in a flow passage in flowing state, by detecting the reaction along the direction of the passage.

CONSTITUTION: A specimen having immobilized enzyme and containing a luminescent substance is passed through a transparent capillary 1 to effect the emission of light by the reaction of a specific chemical substance in the specimen with the immobilized enzyme. The light intensity is detected by the photo-detector 2 moving along the length of the capillary 1 by the driving device 3. The signal proportional to the light intensity is amplified with the amplifier 4 and inputted to the arithmetic unit 5, in which the rate of increment of the light intensity is calculated or the zero increment of the light intensity is detected. The result of end assay is outputted and processed by the external inputting device 6 and the processor 5 and each result is displayed on the display 7.