To provide a hydraulic rotating device, obtaining large turning torque even if the axial dimensions are small, and stably holding an angle even if a member to be rotated is turned to a predetermined angle and suspended.

In this hydraulic rotating device, cylinders 3 to which a piston 4 provided with a ball 6 at the head part is fitted are formed at designated pitches in the outer peripheral part of an inner member 6, the inner peripheral part of an outer member 10 is provided with a corrugated cam face 11 where the balls 6 roll, and an oil pressure circuit 21 is provided for supplying and recovering hydraulic oil to cylinder chambers 3a, 3b opposite to the respective slants of the corrugated cam face. The inner member is provided with a lock pin 32 energized to project toward the outer member 10 by repulsive force of an elastic body 33, and the outer member is provided with an engagement hole 35 where the lock pin engages and disengages. A lock pin retreat oil passage 36a connected to the supply side of the oil pressure circuit 21 is provided to retreat the lock pin.