PURPOSE: To generate plastic deformation locally and to erase the residual tensile stress in the circumferential welded joint of pipes easily by applying reiterative impact forces upon the inside surface of said joint by using a hammering device provided to a rotary bracket.

CONSTITUTION: A rotary bracket 18 is provided to the preceding end part of a rod 6 which is rotated and moved laterally by driving devices 10, 12 via a gear 11, splines 7, a pinion 13 and a ring gear 8. A hammering device 21 consisting of plural air hammers is provided to said bracket. The bracket 18 is in contact with the inside surface 1' of pipe 1 in a roller bearing 20 provided via a rubber cushion 19 on the outside circumferential surface. Compressed air is supplied from a driving power source 15 through a swivel joint 9, the driving power transmitting passage 16 in the rod 6 and intermediate cut wells 17 to the above- mentioned device 21 which in turn applies blowing forces upon the inside surface 1' of the pipes by means of hammering rods 25 to erase the residual tensile stress in the welded joint part and further to form residual compressive stress and to smooth the inside surface 1", whereby the flow resistance in the pipe is decreased.