A wire harness manufacturing method prevents inadvertent deformation of thermoplastic material and separation of thermoplastic material. A predetermined part of an electric wire 91 is accommodated in a through hole of a tubular body formed by connection between a first and second nest members (123, 124) of a nozzle (12), by integrally connecting first and second case body members (121, 122) of the nozzle (12), with the predetermined part of the electric wire 91 therebetween. An approximately tubular covering member (92) covering the predetermined part of the electric wire (91) is molded integrally with the thermoplastic material, by discharging thermoplastic material plasticized by a material plasticizing unit (11) from thermoplastic material discharge orifices (1213) and (1223) in the nozzle (12) to the outer periphery of the electric wire (91), while moving the electric wire (91) and the nozzle (12) relatively to each other.