To provide a belt driving device using a durable wide endless belt which is of simple constitution and can stably be driven to rotate without any belt deviation.

The wide endless belt 200 formed by arranging a core body layer having a large coefficient of elasticity at the breadthwise center part and an elastic body layer 220 having a smaller coefficient of elasticity than the core body layer 210 to the overall width or at least at one side is extended among a plurality of rotation members 300 and driven to rotate and at least one end part of at least one rotary member 300 is formed into a large-diameter part 302 having a larger diameter than the center part and the large-diameter part 302 is positioned inside the elastic body layer 220 at an end of the wide endless belt 200.