When connecting a blood collecting unit (not shown) with an adaptor, streaming blood in a first inflow passage, and streaming liquid including surfactant or fluorescent dyes in a second inflow passage, the blood and the liquid are mixed at a mixing portion, and platelets, erythrocytes and cell membranes of leukocytes in the blood are dissolved by the surfactant, and the nuclei of the leukocytes are fluorescently stained with the fluorescent dyes. Then, a fluorescent particle counting portion counts the fluorescently stained nuclei of the leukocytes during moving in an outflow passage. According to the invention, a number of leukocytes can be correctly measured with a simple operation, such as connection of a blood collecting unit or the like with the adaptor.