(1) preparing a template that is made of a template forming curable resin and has a concave portion corresponding to an optical waveguide core convex portion, and that is provided with through holes which extend at respective ends of the concave portion from the concave portion through to a surface of the template opposite to a surface having the concave portion thereon and which are, respectively, a through hole having a function of a liquid reservoir and another through hole for performing vacuum suction;

(2) applying an ozone treatment or irradiating light having a wavelength of 300 nm or less to at least one of the surface of the template having the concave portion and a core formation surface of a cladding film substrate wherein the cladding film substrate is an alicyclic olefinic resin film;

(3) bringing the cladding film substrate into close contact with the template such that the surface of the template having the concave portion and the core formation surface of the cladding film substrate surface directly contact each other, the at least one of the surface of the template having the concave portion and the core formation surface of the cladding film substrate having been subjected to the application of the ozone treatment or the irradiation of light in step (2);

(4) filling in the core forming curable resin into the concave portion of the template by applying suction from the through hole for performing vacuum suction;

(5) curing the filled core forming curable resin to form a core;

(6) removing the template from the cladding film substrate; and

(7) forming a cladding layer on the cladding film substrate on which the core has been formed.