PURPOSE: To high accurately form a groove in desired depth to the peripheral surface of a pipe body, by inserting a center roller into the pipe body and providing slit rollers corresponding to the center roller to the outside of the pipe body.

CONSTITUTION: A center roller 1 supported by a bearing to the side of a frame main unit and a pair of slit rollers 10 supported by a bearing to a branch side are reversely moved while approximated to and separated from internal and external surfaces 25, 27 of a pipe body A simultaneously by rotating a screw lever 19. Protrusive blades 15, 15', 15" for forming a groove are provided to the roller 10 while recessed parts 6, 6', 6" corresponding to these blades are formed to the roller 1. The both rollers are forcibly contacted, and the groove is formed to the peripheral surface 27 of the pipe body A.