An outer periphery of a single core conductor having a round rod shape and configuring an electric wire is coated with an insulation film such as enamel, except for a partial conductor exposed part at an end portion of the single core conductor, an end portion of the single core conductor is inserted into a through-hole of a seal member made of an elastic material that is mounted to a waterproof casing with being closely contacted thereto, an inner periphery of the through-hole of the seal member is in close contact with an outer periphery of the coated insulation film, and a connection terminal is provided to the partial conductor exposed part at the end portion of the single core conductor.