An attachment mechanism of an optical scanner includes a supporting structure with an opening and a fixing member. The fixing member positions and fixes the optical scanner with a positioning boss held by the opening. The fixing member includes an annular part having an outer circumference face fitted in the opening and an internal circumference face fitted on the positioning boss. One of the internal circumference face and an outer circumference face of the positioning boss includes a projection and another includes a depression for insertion of the projection. A top face of the projection and/or a bottom face of the depression slope(s) slanted from the circumference. By inserting the positioning boss in the opening, fitting the annular part in the opening, and turning the fixing member to narrow a gap between the top face and bottom face, the optical scanner is fixed to the supporting structure.