A method of forming a circuit board which includes generating laser light with a carbon dioxide laser and making a hole through an insulating substrate by irradiating the insulating substrate with the laser light. The hole includes a top opening in a top surface of the insulating substrate, a bottom opening in a bottom surface of the insulating substrate, and an inner wall extending from the top opening to the bottom opening along a thickness direction of the insulating substrate, the inner wall including a bulge which extends in a direction generally orthogonal to the thickness direction. A via hole is formed in the insulating substrate by providing metal in the hole such that the metal extends from the top opening to the bottom opening along the inner wall, and completely closes each of the top and bottom openings.