amplitude corresponds to the change of mass balance in rotating disk 15 shown in fig1 the vibration amplitude in the horizontal direction increases according with the increase of the mass unbalance when the polygon mirror deviates from the original position as shown in fig4 a when the surface roughness ry at upper end surface 18 c of flange 18 is not lower than 3 u03bcm good results in which the changes of the vibration amplitudes are small are obtained in every cases on the contrary when the surface roughness ry is not greater than 1 u03bcm the changes of the vibration amplitudes become considerably large further the inclination angle of the mirror surface of the polygon mirror in each light beam deflecting apparatus is measured by irradiating laser beam onto the mirror surface fig4 b shows the measuring results of the inclination angles as shown in fig4 b when the surface roughness ry at upper end surface 18 c of flange 18 is not greater than 20 u03bcm good results in which the inclination angles are not so large are obtained though the inclination angle should be zero in an ideal state on the contrary when the surface roughness ry exceeds 20 u03bcm the inclination angle is getting considerably large next as the second embodiment of the present invention a method for manufacturing the light beam deflecting apparatus shown in fig1 will be detailed in the following fig5 a 7 b show explanatory illustrations for explaining various methods of finishing upper end surface 18 c of flange 18 which is.