PURPOSE: To form easily a pipe joining part into a thick-walled excellent shape without deteriorating its quality, by expanding the mouth of a pipe by heating and pressing down it with the aid of a tool whose annular clearance between concentric inner and outer diameter cores are formed into the guiding and expanding parts having prescribed depths and thicknesses respectively.

CONSTITUTION: In an annular clearance 8 between concentric inner and outer diameter cores 6, 7 of a tool 4; the core 6 is formed into the dimensions of a part 6a to be fitted to the inner diameter of a pipe and a part 6b to be fitted to the outer diameter of the pipe, and the inner diameter of the core 7 is formed into the dimension of a bulging part of the pipe. Further, the depth of the clearance is conformed to the depth required for the expanding part. Thus, the expanded pipe-end having a desired shape is obtained by pressing down the heated mouth of pipe 1 with the aid of this tool 4. Further, the expanded part of pipe is brought into well contact with an inserted pipe-end without producing a gap between them, when the stepped part of core 6 is formed into a rectangular shape.