An apparatus for discriminating an optical signal from others comprises a circuit for adding one of discriminating signals to a corresponding optical signal. The discriminating signal is of a frequency lower than a band of the optical signal. Therefore, the discriminating signal is easily detected by being passed through, for instance, a low-pass filter.An apparatus for tuning an optical wavelength filter used in apparatus for discriminating an optical signal from others comprises a circuit for frequency-modulating an optical signal by a first discriminating signal and for amplitude-modulating the optical signal by a second discriminating signal having a phase information of the first discriminating signal. An optical wavelength filter is controlled to be tuned to a predetermined wavelength in accordance with a difference between a phase of the first discriminating signal and a phase of a first discriminating signal produced by the second discriminating signal, respectively, included in an output signal of the optical wavelength filter.Alternatively, the apparatus for tuning an optical wavelength filter used in an apparatus for discriminating an optical signal from others comprises a circuit for modulating an optical signal by a discriminating signal of a predetermined period. The optical wavelength filter is controlled to be tuned to a predetermined wavelength in accordance with a difference between a phase of the discriminating signal and a phase of a discriminating signal included in an output signal of the optical wavelength filter.