A leukocyte detector comprises a first and a second filter means adapted to select color components with different wave lengths, or green and blue color components, respectively, from the microscopic image of a stained blood smear, a first and a second opto-electric converter means for converting the transmitted light from said first and second filter means into corresponding electrical signals, a subtractor for subtracting the outputs from said first and second opto-electric converter means, and a comparator which compares the output of said subtractor with the predetermined threshold limit value.