To provide a rotary pile driver preventing a mast from moving in the turning direction even if a big friction resistance acts on a driven pile by fixing the mast with a chuck.

The mast 16 is turnably provided on a base 12 of the rotary pile driver 10. The chuck 18 for holding a steel pipe sheet pile 1 is provided on the mast 16 vertically movably. A circular arc shaped rack 24 is fixed on the lower rear section of the mast 16. A pinion gear 26 is turnably installed on the base 12. The pinion gear 26 and the circular arc shaped rack 24 are meshed with each other. A hydraulic motor 28 is placed in the center section of the pinion gear 26.