To provide a pipe expanding device for efficiently manufacturing a heat exchanger by integrating a multiple superposed heat radiation fin and a plurality of pipes inserted into the heat radiation fin by press-fitting a pipe expanding mandrel to expand the pipe into the pipe.

An operating plate 6 having a plurality of the pipe expanding mandrels 7 and provided freely ascendably/descendably, a pair of first guide bars 10 at the lateral position on the front side of an end plate 9, and a pair of second guide bars 11 at the lateral position on the rear side, are respectively provided apart between a base part 2 and a ceiling part 4. At least either of the front side or the rear side of the end plate 9 is mounted freely slidably on at least either of the first guide bars 10 or the second guide bars 11. A guide passage 2a of a conveying body 23 to transfer the heat exchanger is provided on the base part so as to be passable.