To provide an automatic aligning pulley capable of preventing meandering of an endless belt and to provide a belt driving device provided with the pulley.

This belt driving device 1 has a pair of automatic aligning pulleys 10, and the endless belt 100 is stretched between the pair of automatic aligning pulleys. The automatic aligning pulley 10 on one side functions as a driving pulley and is provided with a driving source 80 such as a motor. The automatic aligning pulley 10a on the other side functions as a driven pulley. Two automatic aligning pulleys 10 are supported by a bearing device (not shown). A shaft 20 supports a drum 30 through an aligning joint device 40 so as to swing freely. An inner ring 42 has a curved channel 44, and a tip part 48 of a bolt 46 screw-fitted into an outer ring 50 is inserted into the channel 44.