An object is to manufacture an optical waveguide having low optical loss, by smoothing the surface of a core. To this end, a method for manufacturing an optical waveguide includes: a core-forming layer formation step of forming a core-forming layer of a photosensitive polymer on a surface of a lower cladding layer formed on a substrate; a smoothing step of smoothing the surface by lowering a surface viscosity thereof through a heat treatment of the core-forming layer; and a photocuring step of forming a core through selective exposure of the smoothed core-forming layer.