PURPOSE: To secure the stable looper tension at abnormal time by switching the looper tension control from tension control mode to electric current control mode in the case abnormality is generated in tension measured value.

CONSTITUTION: A metal strip S is introduced into a deflector roll 11 and is in a looper. At normal case, a speed controller 14 and electric current controller 15 are sequentially controlled by the control signal ΔT of a tension controller 13 so as to keep a tension of the strip S is constant. When a abnormal discrimating device 21 discriminates the signal of tension meter 12 of defector roll 11 as breakage, a switching relay 22 is actuated to turn contacts 22a, 22c to ON and contacts 22b, 22d to OFF and then the output of tension controller 13 is interrupted. A tension setting value Ts is inputted in a tension/electric current transforming device 23, an electric current command value Is* is outputted to an electric current controller 15 as a new electric command. By this method, the secondary breakage in looper is prevented.