PURPOSE: To prevent the deflection by detecting the deflection of a belt and forming a rotary magnetic field in the reverse direction and allowing said magnetic field to act onto magnetic wire ropes buried into the belt and a non- magnetic secondary inductor and generating the movement in the opposite direction to the deflection.

CONSTITUTION: When a belt 1 travels at a normal position, each position of the wire ropes 8L, 8C, and 8R corresponding to detection coils 18L and 18R is symmetrical, and deflection is not detected. Because of the deflection of the belt 1, the relative relation between the detection coils 18L and 18R and the wire ropes 8LW8R is changed, and a difference is generated in the sensitive output, and the deflection of the belt is detected, and the applied voltage onto a rotary magnetic-field apparatus and the direction of the magnetic-field are controlled. Therefore, the magnetic flux due to eddy current is generated in a secondary conductor 9, and acts with the magnetic flux of the rotary magnetic field and transfers the belt 1 in the direction for correcting the deflection. With such constitution, the deflection of the belt can be corrected.