PURPOSE: To enable a stable measurement of a changing process or the like in a reaction system in an FIA analysis by a method wherein a mixture region of a sample and a reaction sample formed in a carrier solution is introduced into a closed channel containing an object to be inspected and caused to be circulated therethrough or stopped or bypassed to measure a reaction product under a circulating or reciprocating condition.

CONSTITUTION: A carrier solution flows in from a conduit a and drains from a drain pipe h with a pump b via an injector d, a reaction coil e, a three-way valve 1, a detector 9 and a three-way valve 2. Under such a condition, as a small amount of a sample is injected into the carrier solution from the sample injector d, it is mixed thereinto to form a mixture region which is sent to the detector g passing through the reaction coil e, a connection pipe f and the three-way valve 1 while it reacts to the reagent in the carrier solution. At the moment when the mixture region flows into the detector g, it is introduced outside the detector by stopping a constant flow pump b or bypassing it while a closed channel comprising the three-way valve 1, a pump 3, the three-way valve 2, the detector g and the three-way valve 1 is formed with a switching of the three- way valves 1 and 2. Then, with the action of the pump 3, the carrier solution containing a mixture region circulates through the closed channel. The measurement under such an agitation can improve the accuracy and reliability.