image obtained by scanning the document must be accurately focused on the surface of the photoconductive drum 32 in accordance with the inherent focal length of the lens 28 however in practice variations in optical characteristics such as focal lengths of supposedly identical lenses variations in component parts of the machine and or variations in the assembly process are inevitable and cannot be prevented as a result an out of focus state occurs on the photoconductive drum 32 in order to compensate for the out of focus state the operator such as a factory worker properly and manually sets the minute displacements l1 and l2 of the lens 28 and the mirrors 24 and 26 by using the first to third dip switches 108 110 and 112 while visually observing and checking the out of focus state of the optical image on the actual photoconductive drum 32 in accordance with a trial and error procedure in order to fine adjust the position of the lens 28 the operator can adjust the two dip switches 108 and 110 the displacement set by the second dip switch 110 is more precise than that set by the first dip switch 108 so that the operator can fine adjust the position of the lens 28 in accordance with the switches 108 and 110 the data 114a and 114b from these switches 108 and 110 are supplied to the microprocessor 100 and a control signal 120 is supplied to the lens drive motor 92 under the control of the microprocessor 100 the motor 92 starts rotation in response to the signal 120 so that the lens block 88 is driven so as to move the lens 28 by the distance corresponding to the data 114a and 114b the data 116.