A vent hole of a mold for molding a tire is formed by suppressing bending deformation of a wire in a casting mold. By using a relational expression among a length of a wire in a casting space, a diameter of the wire, a contact angle of the wire with respect to a molten metal, and a bent amount of the wire by casting acquired by experiments, the contact angle of the wire at which the bent amount of the wire is within an allowable range is calculated from conditions of an actual length and the diameter of the wire. On the basis of the calculated contact angle of the wire, the wire is disposed in the casting space. By pouring the molten metal into the casting space, a cast metal with the wire cast-in in the casting space is cast. A vent hole is formed by withdrawing the wire.