To reduce the effects of the contaminants in the measurement of microorganisms and the reduction of the time necessary for the measurement. Measurement is performed of the microorganism prior to and following culture, and the difference between the two is found. This prevents errors caused by the effect of contaminants contained in the specimens. Since the measurement of the microorganism is performed by means of a flow cytometer, the microorganisms can be measured even when the culture period is short. Moreover, the measurements are accurate, since the contaminants are not measured. Furthermore, the growth form of the microorganisms can be determined by measuring the changes in the intensity of the light emission over the duration of emission of the forward scattered light detected by means of a flow cytometer. Accordingly, based on differences in the particle-size distribution prior to and following culture, it is possible to formulate five major bacterial classifications: Bacilli, Staphylococci, Streptobacilli, Streptococci and yeast fungi.