Carbon steel pipes are joined to each other in a non-oxidizing atmosphere by using an insert constituted by a Ni-base alloy or a Fe-base alloy having a melting point which is lower than 1200°C and a thickness of 80mm or smaller under conditions that the surface roughness Rmax of the joined surfaces of the carbon steel pipes is 20 µm or smaller, the joining temperature is not lower then 1250°C nor higher then 1330°C, duration at the joining temperature is 30 seconds or longer and the pressure is not lower than 2 Mpa nor higher than 4 MPa. The expanding operation is performed such that the maximum difference in level of the joined portion is not larger than 25% of the carbon steel pipe and the expansion ratio is 25% or lower.