PURPOSE:To prevent the diameter of a spot on a scanned face from being changed in main scanning and sub-scanning directions by covering the reflecting face of a deflector with a specific slit member and rotating the slit member together with the reflecting face. CONSTITUTION:A cylindrical slit member 29 is applied to a pyramidal mirror 24 for the deflector 23 and fitted so as to be rotated unitedly with the mirror 24, a square slit 30 is formed on the bottom part 29A of the member 29, and when it is defined that the diameter of a necessary beam in the main scanning direction (x) is (a) and the diameter of the beam in the subscanning direction (y) is (b), the size of the slit 30 is formed so as to be the same as the diameter (a). On the other hand, an elliptical slit 31 is formed on the outer periphery of the member 29 and the size of the slit 31 is set up so that the diameter in the subscanning direction (y) corresponding to a short axis is formed by the size (b) and that in the main scanning direction (x) corresponding to a long axis is formed by the diameter larger than the size (a). Consequently, a uniform beam spot can be formed on the scanned face.
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