PURPOSE: To obtain a flow injection analytical method capable of performing accurate measurement, by a method wherein absorbancy is measured by light having a color other than that of a reaction liquid and the absorbancy of the reaction liquid is measured by light having the color same to that of the reaction liquid when the measured absorbancy reaches zero.

CONSTITUTION: The change in quantity of light is monitored by a peak monitor circuit 16 by using light other than a red color, for example, by blue light as the light from a spectrophotometer 9. In this case, as a reaction liquid is flowed to a cell, the quantity of light is changed as shown by a curve. Because the blue color different from the red color being the color of the reaction liquid is used, the measurement of a light absorbing amount due to the reaction liquid itself is not participated at all. As mentioned above, blue light is changed over to the red color in a wavelength change-over circuit 17 at the point of time when absorbancy is zero in the change in the quantity of light due to blue light, that is, at the point of time (s) when lens action is not acted at all in the leading end part and the trailing end part of the reaction liquid. This measured value is the actual measured value of the reaction liquid.