To provide a piping method which simplifies a dimensional adjustment of a copper pipe at a piping site without previously applying processing to the copper pipe, and which prevents the copper pipe from slipping off a joint after cutting the copper pipe.

A copper pipe 20 is passed through an inner diameter of a coupling joint 1 and is then slid at a piping site, and a rib-shaped projection 21 is molded along an outer periphery of the copper pipe 20 by a projection molding tool so as to configure a union socket allowing the copper pipe 20 to exhibit a slipping-off prevention function.