To provide a slip detection method for a roller straightener by which the surface of a material can be prevented from being changed in quality by a slip between straightening rollers and the material during straightening materials of round steel or a steel pipe with two-roller straightener.

In the method of detecting the slip in the roller straightener, the load currents of the rollers are monitored at all times to catch current fluctuation when the slip is caused, then a slip alarm is issued to automatically stop the straightener when the material during straightening gets out of the straightening rollers.