An optical quantizer including: a first shaping unit which performs at least intensity modulation of the sampling optical pulses using an analog signal to generate first optical pulses having a spectrum in which intensity is flat in a spectrum axis direction; a second shaping unit which performs spectrum shaping of the sampling optical pulses to generate second optical pulses having a spectrum in which intensity increases or decreases monotonously in the spectrum axis direction; a phase shifter which shifts a phase of optical pulses input to one of the first shaping unit and the second shaping unit so that a phase difference between the first optical pulses and the second optical pulses becomes a predetermined phase difference; an interference device which causes interference between the first optical pulses and the second optical pulses; and a wavelength demultiplexer which demultiplexes optical pulses output from the interference device into light of wavebands.