PURPOSE: To increase the information volume by arbitrarily shifting up or down the wavelength of transmitted optical information to the longer wavelength side as well as the shorter wavelength side in a wavelength converting device which converts the wavelength of optical information which has wavelengths multiplexed and is transmitted through an optical fiber transmission line.

CONSTITUTION: Optical switches 121, 122... 12m which switch the outputs according optical information having wavelengths λ1, λ2...λm which are demultiplexed by wavelength should be outputted as they are or after conversion to the longer wavelength side or the shorter wavelength side, a wavelength up-shift register 13 which stepwise shifts the wavelength of optical information inputted through optical switches 12 to the longer wavelength side up to a desired wavelength, and a wavelength down-shift register 14 which stepwise shifts the wavelength of this optical information to the shorter wavelength side up to a desired wavelength are provided. Many stages of wavelength one-step up elements U and optical switches S are connected to constitute the former register, and many stages of wavelength one-step down elements D and optical switches S are connected to constitute the latter.