PURPOSE: To miniaturize a reflection mirror at the time of carrying, reduce the cost for forming, and improve the accuracy by cutting the reflection mirror formed to one body into plural reflection mirror elements and joining these reflection mirror elements again in cut positions to form the reflection mirror.

CONSTITUTION: A reflection mirror 16 provided with through holes 7 is cut into plural reflection mirror elements 2a∼2d by a cutter. It is cut along ribs 17, and ribs 17 are divided in middle parts in breadthwise directions to form ribs 6 for rejoining. Plural reflection mirror elements 2a∼2d formed in this manner are packed in a packing box 21 as a set of reflection mirror members 1 of an offset parabolic antenna and are transported to a shop and are sold. Reflection mirror elements 2a∼2d are transported to a place, where the offset parabolic antenna should be installed, in this state and are joined again in cut positions there to obtain the reflection mirror. Thus, the reflection mirror is miniaturized when being transported, and the cost for forming is reduced, and the accuracy is improved.