A bonding method for a conductor of an electric wire includes a conductor formed of a plurality of strands and a sheath covering the conductor such that the conductor is exposed to a predetermined length. The bonding method ultrasonically bonds the plurality of strands of the electric wire to each other using an anvil and a horn. When the strands are ultrasonically bonded to each other by clamping a part of the conductor exposed from the sheath between the anvil and the horn throughout a predetermined length and ultrasonically vibrating the horn, a distance from the anvil or the horn to the sheath of the electric wire is shorter than a length of the strands when the strand vibrates in a primary mode by ultrasonic vibration.