To provide a caulking apparatus capable of caulking a small-diameter pipe by swelling and deforming the pipe from inside toward outside.

The caulking apparatus 10 is provided with a split-die bulge 12. The bulge 12 is made nearly cylindrical by assembling four split-dies 12a, which are identical in shape. On each of split-dies 12a, a groove 12a4 is formed which constitutes an annular groove of the bulge 12, in the side of a caulking claw 12a3, and where a groove 12a5 is formed at the entering side of a shaft 14. An O-ring 12a6 and an O-ring 12a7 are, as annular elastic members, mounted in the groove 12a4 and the groove 12a5 respectively in such a state that the bulge 12 is formed by assembling each of the split-dies 12a. Thereby an excessive load on the bulge 12 (namely, split-dies 12a) that is partially generated in caulking is reduced, so that a pipe P is joined to a flange F by swelling and deforming (radially expanding) the pipe P.