A method for producing a three-dimensional (3D) object having excellent moldability and mechanical characteristics is provided. The method includes a molding step of irradiating a composition filled in the cavity of a mold with electromagnetic waves having a wavelength of from 0.01 m to 100 m, and molding the composition into the 3D object. The composition for molding a 3D object contains a solvent and at least one of a polymer and a polymerizable monomer.