PURPOSE: To facilitate the assembly of a rotary polygon mirror and to simplify the its constitution by inserting a shaft or bearing into the center hole of the rotary polygon mirror and fixing it by a jig, injecting an adhesive into the gap part between the center hole of the polygon mirror and shaft or bearing in this state, and solidifying it and uniting them.

CONSTITUTION: The shaft 2 for rotating the polygon mirror which has plural reflecting surfaces at the outer periphery is inserted into the center hole (h) of the polygon mirror 1, and the respective members are fixed by the jig 5 so as to obtain necessary relative positions and accuracy. In this state, the adhesive4 4 such as epoxy resin is injected into the gap 3 and solidified to unite the polygon mirror 1 and shaft 2; and the jig 5 is removed to form the rotary polygon mirror. Consequently, the rotary polygon mirror is formed at low cost and a high-accuracy optical deflector which has small surface inclination because of the constitution is obtained.