A measuring signal in the detecting area is calibrated on the basis of a measuring signal in the reference area by the reference method. The rate of change with time of the difference between the measuring signal and an estimated result of the measurement (ORU constant) is calculated in the period where it is expected that the signal shows a fixed value and the components which change at a rate of change calculated for the entire period of the measuring period with the instance when a buffer containing thereon an analyte is supplied taken as a reference are taken as the drift components and correction to remove the drift components from the measuring signal is carried out.