Manufacturing method of optical waveguide

A manufacturing method of an optical waveguide, comprising:

• a step of preparing a workpiece which includes a core layer including a core portion-forming region and has a sheet shape;
• wherein the core layer satisfies all of the following requirements (a), (b), and (c),
• (a) the core layer contains a polymer and a monomer, and has a refractive index distribution based on a difference in concentration of the monomer or a difference in concentration of a structure derived from the monomer,
• (b) the core portion-forming region includes a plurality of first core portions, second core portions provided on both sides of the first core portion, and a first side cladding portion provided between the first core portion and the second core portion, and the core portion-forming region further includes a plurality of second core portions positioned at opposite ends of the core portion-forming region in a direction perpendicular to a longitudinal direction of the first and second core portions,
• (c) the core portion-forming region is a region where a total area ratio occupied by the first core portion and the second core portion is 50% or more in a 1 mm-square range, and
• a step of cutting out an optical waveguide from an inside of the core portion-forming region in the workpiece so as to avoid at least two of the second core portions positioned at each of the opposite ends of the core portion-forming region.