A signal separation system is used to discriminate cytoplasm of white blood cells from a sample of white blood. The signal separation system includes a light source generating light passed through a blood sample, a light separator for separating green and blue light beams from the light passed through the sample, first and second photoelectric converters for changing the green and the blue light beams to first and second electric signals, a signal treatment circuit which converts the first and the second electric signals to binary signals with respect to certain threshold levels and then forms a separation signal from the binary signals, and the gate circuit which separates the signal component of the white blood cells from the sample with the function of the separation signal.