To correct the external surface of an incomplete product to the size and shape within the allowable range at a very high speed by advancing/ retracting one set of rollers in a housing to be rotated around the axis with respect to the axis, and simultaneously bringing the rollers into contact with a product moving through a guide along the axis.

A product W such as a steel rod and a wire to be fed out of a steel mill are fed into a guide 66, and the product W is passed through a curved part between four rolls 98 fitted to a roller assembly 90 fitted with equal intervals around the axis in a housing 60 to be rotated around the axis. The roller 98 is of a specified external shape to form the product W passing therethrough to a desired shape. A collar 122 is rotated by a drive motor, and the collar 122 is moved in the axial direction within the housing 60 along a threaded surface 120, and directed to a bearing pad 104 of the roller assembly 90, and tightens the roller 98 inwardly so as to be brought into contact with the product W.