A wavelength converting device has a substrate made of an electro-optic material and converts a wavelength of a fundamental light to oscillate a converted light. A wavelength converting portion is provided in the substrate and has a cross sectional area of 0.0001 mm.sup.2 or larger and 0.01 mm.sup.2 or smaller. A pair of thinner portions are provided in both sides of the wavelength converting portion, respectively, and thinner than the wavelength converting portion.