PURPOSE: To form a 1st and a 2nd antenna using one main reflecting mirror in common and to attain miniaturization by arranging a hyperbolic subreflecting mirror and an elliptic subreflecting mirror sequentially on the ray path of a main reflecting mirror at prescribed intervals and adjusting the movement of the hyperbolic subreflecting mirror via the elliptic subreflecting mirror.

CONSTITUTION: When a hyperbolic subreflecting mirror 11 is subject to moving control to a 1st position while a reflecting mirror drive section 12 is subject to drive control, the hyperbolic subreflecting mirror 11 is formed to a side feeding type Cassegrain antenna opposite to a 1st primary radiator 13 and a main reflecting mirror 10. Moreover, the reflecting mirror drive section 12 is driven inversely to move the hyperbolic subreflecting mirror 12 to a 2nd position, then the hyperbolic subreflecting mirror 11 is escaped from the 1st position and the ray path of the main reflecting mirror 10 and moved to the 2nd position, the elliptic subreflecting mirror 14 is made opposite to the main reflecting mirror 10 and the elliptic subreflecting mirror 14 uses a 2nd primary radiator 15 and the main reflecting mirror 10 to form a Gregorian antenna. Thus, the Cassegrain antenna and the Gregorian antenna with different antenna characteristics using the main reflecting mirror in common are formed.