A differential hydraulic motor comprises a swash plate joined to an output shaft, and a cylinder block joined to an input shaft. A change-over valve is provided which changes over according to the relative rotation of the output shaft and input shaft, and leads a different fluid pressure to pistons housed in the cylinder block according to whether these pistons are extending or contracting. A rotary distributor is provided which connects a high pressure port and low pressure port in a casing to the change-over valve even when the cylinder block is rotating. This rotary distributor comprises a port block which is free to slide in the casing. Pressure from the high pressure port acts on a piston formed on an end face of the port block so as to push the port block against a sliding surface of a cover block joined to the cylinder block. Annular ports connected to the high pressure port and low pressure port are formed on this sliding surface, and high pressure and low pressure are led to the change-over valve from these annular ports.