To provide a rotary driving system capable of obtaining rotary force without using an engine, a motor or the like in a system rotatably driving various driven bodies.

A plurality of rotary body units 3A to 3C each are provided with a balance weight 5 capable of moving back and forth to a direction orthogonal to a rotary shaft 2 and a fluid cylinder 6 moving the balance weight back and forth, and are installed on the rotary shaft 2, in which the rotary body units are shifted by a designated angle in a peripheral direction of the rotary shaft 2. The rotary driving system obtains rotary force by moving the balance weights 5 of the rotary body units 3A to 3C in sequence by the fluid cylinders 6. Further, the fluid cylinders 6 of the rotary body units 3A to 3C are supplied with a fluid fed from a fluid supply source in sequence via a rotary switch valve 10. The rotary switch valve 10 is provided on the rotary shaft 2.