A kit is disclosed for preparing a measurement sample for detecting live cells, injured cells, VNC cells and dead microorganism cells in a test sample by the following steps: a) the step of treating the test sample with an enzyme having an activity of decomposing cells other than those of the microorganism, colloidal particles of proteins or lipids existing in the test sample, b) the step of treating the test sample with a topoisomerase poison and/or a DNA gyrase poison. c) the step of treating the test sample treated in the steps a) and b) with a nuclear stain agent, and d) the step of detecting the microorganism in the test sample treated with the nuclear stain agent by flow cytometry.