Patent ID: 8534852

Claim:
A mirror unit of an exposure device comprising: (a) a mirror in which a longitudinal direction thereof is arranged in a scanning optical axis direction, having support points including one support point on one end portion in the longitudinal direction thereof and two support points at two portions in a width direction perpendicular to the longitudinal direction on the other end portion thereof; and (b) a mirror base member that holds the mirror in a way to support the mirror at the three support points, wherein a line that is formed by connecting the support point on the side of the one support point and one of the support points on the one side of the two support points is made a mirror rotary axis, wherein the mirror is three-point supported on the mirror base member to be capable of adjusting an angle under a condition that the other support point on the side of the two support points is made to be an angle adjustment input point, wherein the mirror is arranged so that the longitudinal direction thereof is inclined for the scanning optical axis in the direction of the mirror width, and the support point on the side of the one point support is set at a center in the mirror width direction, and the support point on the side of the one point support and the one support point on the side of the two support points are set on the scanning optical axis, thereby aligning the mirror rotary axis on the scanning optical axis, and wherein the mirror is arranged so that a center of gravity of the mirror is positioned within a triangle area that is formed by connecting the three support points.