To provide a reflecting mirror expanding mechanism which is capable of finely adjusting not only an expansion angle of orientation around the expansion axis of an reflecting mirror but also around an axis vertical to the expansion axis by the use of a single mechanism of simple structure.

A reflecting mirror expanding mechanism is equipped with a stationary-side expanding mechanism 1, a reflecting mirror-side expanding mechanism 2 mounted with a reflecting mirror 6, and an expansion shaft 11 which supports the reflecting mirror-side expanding mechanism 2 in a freely rotatable manner to the stationary-side expanding mechanism 1. Two means 5 make a part 3 of the stationary-side expanding mechanism 1 and a part 4 of the reflecting mirror-side expanding mechanism 2 approach each other or part from each other in a direction vertical to the expansion shaft 11, the two are arranged apart from each other by a prescribed distance in the axial direction of the expansion shaft 11, and the two means 5 are set so as to operate separately. The reflecting mirror 6 is used as an antenna mounted on an artificial satellite, and the above means 5 are operated by a command sent from the earth to direct the reflecting mirror 6 of an antenna in an optimal direction.