A blood sample containing leukocytes flows through a flow cell. A light source emits a light beam in a first direction. The light beam incident into the flow cell is scattered by the blood sample as scattered light. A first detector detects an intensity of forward small scattered light out of the scattered light. The first detector is arranged so as to have a maximum angle of a detection angle range for the forward small scattered light stemmed from the first direction at a position inside (outside) of the flow cell, which is 3.1 (4) degrees or less. A second detector detects an intensity of forward large scattered light out of the scattered light. The second detector is arranged so as to have a detection angle range for the forward large scattered light stemmed from the first direction at a position inside (outside) of the flow cell, which falls in a range from 8 to 12 (10 to 16) degrees. A processor classifies the leukocytes based on the detected intensity of the forward large scattered light and the detected intensity of the forward small scattered light.