A method of manufacturing a wire harness including a plurality of wires and a connector with a housing. The method includes arranging the plurality of wires in an insertion hole of an airtight block of the housing to have a gap between the plurality of wires and an inner surface of the insertion hole, supplying a molten resin having a fluidity into the gap through a flow channel in communication with the gap, and solidifying the molten resin inside the space to resin-seal the gap between the insertion hole and the plurality of wires. The supplying of the molten resin is conducted such that a tool for melting a solid resin member is attached to the airtight block, the resin member is melted by applying an ultrasonic vibration while being pressed against the tool, and the molten resin obtained by the melting is poured into the flow channel.