To provide a pipe expanding device capable of simply and surely expanding a pipe.

The pipe expanding device 10 includes a pair of gripping pieces 21, 22 for gripping the pipe 1, a diameter expansion section 31 for expanding the front end 2 of the pipe 1 by press-fitting it to the front end 2 of the pipe 1, and a control component 50 for controlling a projection amount of the front end 2 of the pipe 1 from one end face 23 of the gripping pieces 21, 22. The control component 50 is detachably installed to the gripping pieces 21, 22, and has a blocking plate section 51 for blocking an opening of a pipe insertion hole 24 located at a side where the front end 2 of the pipe 1 is exposed. The blocking plate section 51 includes a butting surface 510 coming in contact with the end faces 23 of the gripping pieces 21, 22, and a recessed section 520 which has a shape depressed from the butting surface 510 so as to make the front end 2 of the pipe 1 project at a designated amount h from the end faces 23 of the gripping pieces 21, 22 and can receive the front end 2 of the pipe 1.