a front plate 47 stands upright in front of the base plate 42 of the stand 41 facing the mill line l a side plate 48 for supporting and reinforcing the front plate 47 is fastened to each end of the front plate 47 a bracket 51 fastened to the front plate 47 swingably supports on a pin 54 an arm 52 with a u shaped roll support 53 a connecting member 55 is swingably attached to the front part of the arm 52 by a pin 56 to the connecting member 55 is coupled the front end of an adjusting screw 57 which is rotatable about the axis thereof a bracket 49 is fastened to the top of the front plate 47 of the stand 41 the bracket 49 has a pin 58 which swingably supports 41 the bracket 49 swingably supports through a pin 58 a sleeve 59 that receives the adjusting screw 57 by rotating the adjusting screw 57 in and out the arm 52 is swung up and down about the pin 54 in the cage roll unit 11 the roll support 53 on the arm 52 rotatably carries a cage roll 72 with a concave surface the curvature of which is identical with that of the piece p being formed the cage roll 72 presses slanting downwardly from above the edges of the piece p toward the round shape toward the center thereof an arm 61 with a u shaped roll support 62 is fastened to the front plate 47 below the bracket 51 the roll support 62 has rotatably mounted thereon a cage roll 73 of the same shape as the upper cage roll 72 this lower cage.