An optical quantizing unit includes an optical divider dividing 1.sup.st optical pulses to be quantized and sending the divided 1.sup.st optical pulses into a plurality of paths; a plurality of optical filters passing with different transmittances the divided 1.sup.st optical pulses; and an optical threshold filter sequentially receiving the 1.sup.st optical pulses, and sending 2.sup.nd optical pulses when light intensities of the 1.sup.st optical pulses are above a preset threshold value.