Disclosed is a through-hole forming method including a process of forming, by condensing and irradiating a laser beam onto an insulation substrate through a lens, a through-hole that passes through the insulation substrate in a thickness direction of the insulation substrate. A medium between the lens and the insulation substrate is air. A converging half angle θ that is calculated from a focal length f of the lens and a beam diameter d of the laser beam that enters the lens by using expression (1) satisfies expression (2): (d/2)/f=tan θ . . .   (1), and 0.16≦sin θ≦0.22 . . .   (2).