PURPOSE: To perform stable tension control even at reversible operation without detecting field current of a generator, by a method wherein electromotive force of the generator is estimated based on output voltage of the generator, and gain compensation is performed corresponding to previously set state.

CONSTITUTION: In a Ward-Leonard drive of a field thyristor type, voltage V of a generator 2 is detected by a voltage detector 6, and inputted together with current I into an impedance correction circuit 11. The impedance correction circuit 11 corrects generator armature resistance and inductance components of the detected voltage value V using actual current I, and signal E therefor is inputted in a gain setting device 12. An ignition control circuit 10 outputs a gate pulse based on the input, and field current IFG flows corresponding to the output voltage of a thyristor converter 1. The generator 2 generates the output voltage V corresponding to the field current IFG, and controls the current I to the main circuit.