In a light deflection device, the radiation efficiency of radiated light beams is to be improved. The light deflection device is configured by a photonic crystal waveguide having a lattice array with low refractive index parts periodically arrayed in a surface of a high refractive-index member. The lattice array has a dual-periodic structure consisting of a first periodic array and a second periodic array which differ from each other in periodic arrangement of the low refractive index parts. A line defect where no low refractive index parts are arrayed constitutes a waveguide core for propagating incident light. The cross section of at least either the first periodic array or the second periodic array constituting the periodic arrays of the dual-periodic structure is asymmetrical in the thickness direction of the low refractive index parts.