It is provided a wavelength converting device oscillating an idler light having a wavelength of 5 to 10 μm from a pump light. The wavelength of the idler light is longer than that of the pump light. The wavelength converting device includes a wavelength converting layer 5 of a semiconductor non-linear optical crystal and having a thickness of 50 μm or smaller. The wavelength converting layer 5 includes a crystal orientation inversion structure wherein crystal orientation of the optical crystal is inverted at a predetermined period and at least one flat main face 5b. The device further includes a Peltier device 2 controlling a temperature of the wavelength converting layer 5; and a clad portion 4 joined with the flat main face 5b of the wavelength converting layer 5 and provided between the wavelength converting layer 5 and the Peltier device 2. The pump light, idler light and signal light satisfies a particular phase matching condition.