In an assembling method of an optical deflection apparatus in which a polygonal mirror is fixed on an outer ring of a radial bearing and an optical scanning is conducted by a rotation of an inner ring of the radial bearing, the assembling method includes the steps of placing the polygonal mirror on the outer ring of the radial bearing wherein the polygonal mirror and the outer ring of the radial bearing have an engagement portion including an adhering portion with a gap having a distance between the polygonal mirror and the outer ring of the radial bearing; adjusting a tilt angle of the polygonal mirror with respect to the outer ring of the radial bearing; and filling an adhesive in the gap of the adhering portion under the adjusted condition, thereby integrally fixing the polygonal mirror and the outer ring of the radial bearing.