PURPOSE: To obtain a large wavelength shift for the light wavelength converting element.

CONSTITUTION: The light wavelength converting element which makes signal light 10 and exciting light 9 having shorter wavelength and larger light intensity than the signal light incident on the same semiconductor optical amplifier, varys the number of carriers of the semiconductor optical amplifier by the exciting light 9, and varies the wavelength of the signal light 10 by using refractive index variation of a semiconductor, has a layer of a semiconductor whose wavelength at its absorption end is longer than the wavelength of the exciting light and shorter than the wavelength of the signal light 10 as one or both of two clad layers 2 and 4 formed across the active layer 3 of the semiconductor optical amplifier.