PURPOSE: To change a conductive roll set without interrupting electrical heating treatment at the time of changing the conductive roll set by arranging plural conductive roll sets adjoined in the advancing direction of a metal strip, at the time of heating the metal strip by Joule heat by conducting the current to the metal strip from the conductive roll set.

CONSTITUTION: A closed circuit is formed with the metal strip 1, conductive roll set 3, conductive member 6 and conductive roll set 5. The metal strip 1 is passed through an annular transformer 7 and AC current is supplied into the annular transformer 7 from an outside electric source 8 and the induction current is generated in the closed circuit and the metal strip 1 is heated by the Joule heat with this induction current. At the time of needing the change of the conductive roll set 5 by wearing caused by contact with the high temp. metal strip 1, the metal strip 1 is held with the prepared conductive roll set 4 and the old conductive roll set 5 is separated from the metal strip 1 to execute the change of the roll set. By this method, the conductive roll set 5 at the outlet side having high changing frequency caused by the wearing can be changed without interrupting the electrical heating treatment.