Pipe joint (30) comprising pipe-shaped body (31) in which pipe (32) is inserted from the rear end side; front ring (33) and back ring (34) fitted round portion of the pipe (32) protruding from the rear end side of the body (31); and cap nut (35) for tightening the front ring (33) and back ring (34) so as to fix the pipe (32) to the body (31). The back ring (34) is constituted of an alloy comprising, on the basis of weight %, 0.001 to 0.01% of C, 5% or less of Si, 2% or less of Mn, 0.03% or less of P, 100 ppm or less of S, 50 ppm or less of O, 18 to 25% of Cr, 15 to 25% of Ni, 4.5 to 7.0% of Mo, 0.5 to 3.0% of Cu and 0.1 to 0.3% of N, the remainder composed substantially of Fe and other unavoidable impurities.