An optical frequency division multiplexer comprising: a nonlinear medium (11, 12) adapted to cross-phase-modulate a carrier light beam with a signal light beam, the signal light beam carrying at least one electric carrier signal at a characteristic frequency (fi) modulated with an information signal (Bi), the non-linear medium thereby being adapted to frequency-multiplex the information signal in the signal light beam on the carrier light beam, to generate an optical frequency division multiplexed signal, and to output the optical frequency division multiplexed signal; and a control unit (15, 19) adapted to control a polarization state of the signal light beam in a direction in which an intensity of a modulation component takes a maximum value, based on the intensity of the modulation component involved in a modulation signal at a desired characteristic frequency of the optical frequency division multiplexed signal, when the intensity of the modulation component involved in the modulation signal at the desired characteristic frequency of the optical frequency division multiplexed signal is reduced beyond a predetermined threshold from a maximum value, wherein the control unit includes: an intensity converting unit adapted to convert a part of the optical frequency division multiplexed signal into an intensity modulated signal; an optoelectric conversion unit that optoelectrically converts the converted intensity modulated signal; an extracting unit adapted to extract the modulation component involved in the modulation signal at the desired characteristic frequency of the optoelectrically converted intensity modulated signal; and an intensity detecting unit adapted to detect the intensity of the extracted modulation component. a nonlinear medium (11, 12) adapted to cross-phase-modulate a carrier light beam with a signal light beam, the signal light beam carrying at least one electric carrier signal at a characteristic frequency (fi) modulated with an information signal (Bi), the non-linear medium thereby being adapted to frequency-multiplex the information signal in the signal light beam on the carrier light beam, to generate an optical frequency division multiplexed signal, and to output the optical frequency division multiplexed signal; and a control unit (15, 19) adapted to control a polarization state of the signal light beam in a direction in which an intensity of a modulation component takes a maximum value, based on the intensity of the modulation component involved in a modulation signal at a desired characteristic frequency of the optical frequency division multiplexed signal, when the intensity of the modulation component involved in the modulation signal at the desired characteristic frequency of the optical frequency division multiplexed signal is reduced beyond a predetermined threshold from a maximum value, wherein the control unit includes: an intensity converting unit adapted to convert a part of the optical frequency division multiplexed signal into an intensity modulated signal; an optoelectric conversion unit that optoelectrically converts the converted intensity modulated signal; an extracting unit adapted to extract the modulation component involved in the modulation signal at the desired characteristic frequency of the optoelectrically converted intensity modulated signal; and an intensity detecting unit adapted to detect the intensity of the extracted modulation component.