In a laser device, a different refractive index region 6B of a photonic crystal layer is arranged at a lattice point position of a square lattice. In the case where a plane shape of the different refractive index regions 6B is a nearly isosceles right triangle, two sides forming a right angle extend along longitudinal and horizontal lateral lines of the square lattice. A direction parallel to or vertical to an oblique side of the triangle and a direction of polarization in the periodic polarization inversion structure of a nonlinear optical crystal NL are the same.