PURPOSE: To improve an antenna device at a low cost corresponding to the change of an applied frequency.

CONSTITUTION: When improving the double reflection mirror antenna system in the case of changing the applied frequency, a primary radiation system parameter is decided so that the radiation pattern of a primary radiator 23b to blow radio waves to a sub reflection mirror 22b can be almost equal to that of the existent antenna system, a phase center 24b of the primary radiator 23b is decided based on this parameter and the newly applied frequency, the coordinate of the sub reflection mirror 22b is decided based on a focal point F of the sub reflection mirror 22b and the coordinate of an existent main reflection mirror 21, and the primary radiator 23b and the sub reflection mirror 22b are produced according to the decided conditions and combined with the existent main reflection mirror 21.