A method for manufacturing a planar optical waveguide device of which a core includes a plurality of alternatively arranged fin portions and valley portions to form a grating structure, in which the core widths of the valley portions vary along the longitudinal direction, the method including: a high refractive index material layer forming step of forming a high refractive index material layer; a photoresist layer forming step of forming a photoresist layer on the high refractive index material layer; a first exposure step of forming shaded portions on the photoresist layer using a phase-shifting photomask; a second exposure step of forming shaded portions on the photoresist layer using a binary photomask; a development step of developing the photoresist layer; and an etching step of etching the high refractive index material layer using the photoresist pattern resulted from the development step.