To reduce size and weight for a rotary drive device using a wave motion gear mechanism.

A piezo-rotor 14 is provided in a rotatable manner inward in the radial direction of a flex spline 12. The flex spline 12 is depressed outward in the radial direction by a long depressing part 14b on the piezo-rotor 14 to engage outer teeth 12a of the flex spline 12 with inner teeth 11c of a circular spline 11. A flexible movable part 14c is formed on the piezo-rotor 14, and a voltage is applied to a piezoelectric element 16 attached to the movable part 14c to generate distortion repeatedly. This deforms the movable part 14c with acceleration repeatedly, thereby generating inertia continuously in the piezo-rotor 14 for rotation. Rotation of the piezo-rotor 14 moves a circumferential position where the outer teeth 12a is engaged with the inner teeth 11c, and rotation of the piezo-rotor 14 is decelerated to be output in the flex spline 12.