A method for counting megakaryocytes in a specimen is described. In the method, first, erythrocytes in the specimen are lysed and nucleic acid in the megakaryocytes is stained with a fluorescent dye, and thereby, a measurement sample is prepared. Next, the cells in the measurement sample are irradiated with excited light so that the forward scattered light, the side scattered light and the fluorescence, which are emitted from the cells, are detected. Megakaryocytes are identified on the basis of the detected forward scattered light, the fluorescence and the side scattered light. Then, the identified megakaryocytes are counted.