PURPOSE: To simplify the device by providing an input shaft capable of freely rotating in the interior of an output shaft having a valve element, switching liquid paths to liquid chambers partitioned by the valve element according to the positional relation between both shafts, and rotating the output shaft by the rotation of the input shaft in the same direction.

CONSTITUTION: When an input shaft 5 is rotated counter-clockwise by the external load, a liquid is supplied to a passage 21, and communicating grooves 20, 22. As the communicating groove 22 is cut off from a liquid chamber 13 by the input shaft 5, the liquid is supplied to a liquid chamber 8 through a passage 25a to force an output shaft 4 to follow the input shaft 5 and rotate. A communicating groove 23 is opened by the input shaft 5, so that the liquid in a liquid chamber 9 is discharged from a discharge port 17 through a passage 25b, the communicating groove 23 and the liquid chamber 13. Accordingly, the output shaft 4 follows the rotation of the input shaft 5 to rotate until the liquid pressure in the liquid chambers 8, 9 becomes equal. When the input shaft 5 is rotated clockwise, the output shaft 4 conducts the following operation in the same manner. As the liquid path selecting valve mechanism is not required, the device can be simplified.