PURPOSE: To set the tension of a work constant by supporting a dancer roller which swings vertically according to the tension through a buffer device and controlling a driving device according to the result of the detection of a sensor for detecting the vertical position of the dancer roller.

CONSTITUTION: A dancer roller 16 is supported in vertically movable ways by a buffer device 20, and the hunting of the dancer roller 16 is prevented. A photentiometer 40 as position sensor is installed onto the dancer roller 16, and the vertical position of the dancer roller 16 is detected by the potentiometer 40. The potentiometer 40 is connected with the torque motor 45 of a taking-up device 4 through a control box 41, and the revolution speed and the torque of the torque motor 45 are controlled so that the tension of a taken-up sheet material S onto a taking-up drum D' is made always constant. Therefore, the generation of hunting in the tension variation is prevented, and a proper tension is always maintained automatically.