To detect belt tension and transmitting force of belt driving by providing a means for electrically detecting distortion of a belt, in a belt driving device including an energizing mechanism for applying tension to the belt.

A contact X is arranged on a driving pulley 1, and a contact Y is arranged on a driven pulley 2, and they are brought into contact with a thin film resistance 5 formed on a belt 3 to be wound. The thin film resistance 5 having a thickness TC and a width WC is formed along a circumference of the belt 3 having a thickness TB and a width WB. In between the contacts X and Y, they are connected to each other through the thin film resistance 5 formed on the belt 3 to be wound, and are an equivalence to parallel connection of two electric resistances. A resistance value between the contacts X and Y is measured, and is calculated using a material value of the belt 3 and the thin film resistance 5, and thereby, belt tension generated by energizing force is found out.