PURPOSE: To adjust a change in the tension of a belt by following it up to a proper value in real time according to a change in rotational speed, and simplify the constitution of this device.

CONSTITUTION: A spindle 3 is rotated with a motor 7 driving through a driving pulley 9, a V belt 10 and a driven pulley 6. The housing 21 of a pressure adjusting mechanism 20 is mounted onto the piston rod 14 of a cylinder 11. A rotating shaft 23 is rotatingly-supported onto a housing 21, and a tension pulley 24 is fixed onto the rotating shaft 23. The tension pulley 24 is engaged with a part of V belt 10. When the rotational speed of the motor 7 changes, the introduction amount of pressurized fluid into the pressure adjusting mechanism 20 from a pump 30 is changed by dynamic pressure change. As a result, pressure P0 applied to the inside of a cylinder chamber 16 in the cylinder 11 from the pump 30 is changed, and the tension pulley 24 is displaced in the tension direction or loosening direction of the V belt 10 through the tension pulley 24.