The invention relates to a numerical controlling unit that includes: a instructing part that provides a command acceleration for a servomotor, a parameter storing part that stores a threshold parameter about a load of the servomotor, and a load measuring part that measures the load of the servomotor. Therefore, possible current leakage caused from the high operating voltage is effectively mitigated, and the performance of the memory device is increased. The reflector is driven according to the value to track the satellite, thereby minimizing a satellite tracking error caused by a sudden movement of a mobile body equipped with the satellite tracking antenna system. Alternatively, a recess may be formed in a portion, facing to each of the upper and lower edges of the outer periphery of the rotating disk portion, of an inner wall surface of the hydraulic bearing.