A first fθ lens is arranged to form an inclination angle α1 to a main scanning plane. A second fθ lens is arranged to form an inclination angle α2. In order to determine the inclination angles α1 and α2, curvature amounts of scanning lines generated on a scanned surface are measured when the fθ lenses are independently inclined one at a time at a minute angle. Change rates K1 and K2 are obtained from the measured curvature amounts and the minute angles. The inclination angles α1 and α2 are determined such that |K1α1+K2α2| is no more than a certain value.