PURPOSE: To smoothly track a satellite by providing an assuming-expression parameter calculating means which estimates the future behavior of the satellite by using the stored past actual angles of an antenna and calculates an antenna driving command value based on the estimated results.

CONSTITUTION: When an actual antenna angle storing/discriminating means 12 discriminates that the step-tacking of a satellite has become impossible, the means 12 stops a tracking permitting signal outputted to a shifiting-to- memory-tracking discriminating means 16. An actual driving angle storing means 11 stores driving command values obtained from an antenna driving command assuming expression and an assuming-expression parameter calculating means 13 calculates the parameter of a driving command assuming expression by using the actual antenna angle data and the parameter is stored 14. An antenna driving command value estimating means 15 calculates a future antenna driving command value from the diving command assuming expression by using the stored 14 parameters. The means 16 shifts the tacking mode to the memory- tracking mode in which an antenna is driven by using the future antenna driving command value when the step-tracking permitting signal is stopped.