A wavelength converting device comprising a light source adapted to emit light with a fundamental wave; and a ferroelectric substrate, wherein a periodic domain inversion structure (3, 4) is formed in said ferroelectric substrate; the periodic domain inversion structure (3, 4) is for converting a wavelength of the fundamental wave to generate a harmonic wave; said ferroelectric substrate comprises an upper face, a bottom face, an incidence face, and an emitting face; the periodic domain inversion structure comprises plural domain inverted parts and plural non-domain inverted parts alternately arranged in a propagation direction with respective vertical domain inversion boundaries between adjacent ones of the domain inverted parts and non-domain inverted parts; the light source is arranged to emit the light such that the light enters the periodic domain inversion structure (3, 4) at the incidence face and propagates through the periodic domain inversion structure (3, 4) in the propagation direction; the vertical domain inversion boundaries are viewed in a cross section parallel with the propagation direction and parallel with a normal line of the upper face of said ferroelectric substrate; respective horizontal domain inversion boundaries between adjacent ones of the domain inverted parts and non-domain inverted parts are viewed in a plan view on said upper face of said ferroelectric substrate; the vertical domain inversion boundaries of said periodic domain inversion structures (3, 4) are inclined with respect to the normal line of said upper face of said ferroelectric substrate, provided that said ferroelectric substrate is viewed in the cross section parallel with the propagation direction and parallel with said normal line of said upper face; and characterized in that said horizontal domain inversion boundaries are inclined with respect to a direction perpendicular to the propagation direction on said upper face of said ferroelectric substrate, viewed in said plan view on said upper face of said ferroelectric substrate. a light source adapted to emit light with a fundamental wave; and a ferroelectric substrate, wherein a periodic domain inversion structure (3, 4) is formed in said ferroelectric substrate; the periodic domain inversion structure (3, 4) is for converting a wavelength of the fundamental wave to generate a harmonic wave; said ferroelectric substrate comprises an upper face, a bottom face, an incidence face, and an emitting face; the periodic domain inversion structure comprises plural domain inverted parts and plural non-domain inverted parts alternately arranged in a propagation direction with respective vertical domain inversion boundaries between adjacent ones of the domain inverted parts and non-domain inverted parts; the light source is arranged to emit the light such that the light enters the periodic domain inversion structure (3, 4) at the incidence face and propagates through the periodic domain inversion structure (3, 4) in the propagation direction; the vertical domain inversion boundaries are viewed in a cross section parallel with the propagation direction and parallel with a normal line of the upper face of said ferroelectric substrate; respective horizontal domain inversion boundaries between adjacent ones of the domain inverted parts and non-domain inverted parts are viewed in a plan view on said upper face of said ferroelectric substrate; the vertical domain inversion boundaries of said periodic domain inversion structures (3, 4) are inclined with respect to the normal line of said upper face of said ferroelectric substrate, provided that said ferroelectric substrate is viewed in the cross section parallel with the propagation direction and parallel with said normal line of said upper face; and characterized in that said horizontal domain inversion boundaries are inclined with respect to a direction perpendicular to the propagation direction on said upper face of said ferroelectric substrate, viewed in said plan view on said upper face of said ferroelectric substrate.