A serum and a reagent are supplied into a reaction tube to prepare a reactive solution or a sample solution which is sucked up at a predetermined position into one of a plurality of flow cells. The flow cells are mounted on a thermostatic block to be heated to a predetermined temperature and alternatively supplied with the sample solution. After a sample solution has been introduced from a specified reaction tube into one of the flow cells and heated to the predetermined temperature, the sample solution in the one flow cell is irradiated with light to measure changes in absorbance of the sample solution for a predetermined time. During a period when the absorbance changes of the sample solution in one flow cell is measured, another sample solution is introduced from another reaction tube into another flow cell and heated to the predetermined temperature. Immediately after the measurement of the absorbance changes of the sample solution in the one flow cell has been completed, the other flow cell is irradiated with light to measure the changes in absorbance of the sample solution in the other flow cell. Thereafter, one and the other flow cells are alternately supplied with a sample solution to successively measure the absorbance changes. Thus, a waiting time for stabilizing the temperature of the sample solution introduced into the flow cell can be actually dispensed with, and therefore effective measurement can be made.