To provide a wavelength conversion element in which it is not needed to set the intensity of a signal light to be inputted exactly and in which the inverting of the waveform of the signal light posterior to the transformation is not present.

The inverting of a waveform is made to be returned by converting a signal light 11 into a signal light 13 having an objective wavelength in two steps through integrating monolithically a first wavelength conversion unit which is provided with a first Mach-Zehnder interferometer 102 to which the signal light 11 is inputted and, also, a first CW light 21 is inputted and which is provided with light amplifiers 103, 104 which are provided in the interferometer 102 and which operates in a state in which the waveform of the light is inverted and a second wavelength conversion unit which is provided with a second Mach-Zehnder interferometer 107 to which a signal light 12 which is converted in the first wavelength conversion unit is inputted and, also, a second CW light 22 is inputted and which is provided with light amplifiers 108, 109 which are provided in the interferometer 107 and which operates in a state in which the waveform of the light is inverted on a substrate 100 so that the conversion units are connected in series.