PURPOSE: To increase the accuracy of measurement, by specifying the suction method of reagent.

CONSTITUTION: The tip of the pipe 3 is inserted in the sample 2 of the test tube 1, and after sunction of a given amount of sample 2 through the operation of piston pump, the sample tube 1 is detached from the pipe 3 and air 8 is suctioned from the sample 2, and the sample 2 is fed to the flow cell 5 with the air 8, and when the rear end of the sample 2 reaches the flow cell 5, the operation of the piston pump is stopped. Thus, the tip of the introduced sample 2 this time where the purity is remarkably lowered through the mixture of the sample at previous time and the remaining sample, passes through the flow cell 5, and the rear end part of the sample 2 high in the purity is located in the flow cell 5. Accordingly, with this state, by the color comparison through the use of the spectrophotometer 7, the accuracy of measurement can be increased.