Focus adjustment of an optical writing unit is performed based on an image of a test pattern formed on a paper sheet. The pattern includes bars of density levels associated with adjustment quantity information indicating numerical values corresponding to the numbers of rotation of adjustment motors and adjustment screws. The density levels decrease as the amount of displacement of the unit with respect to a photosensitive drum increases. A focus adjustment device accepts an input of the numerical values indicated beside the unprinted bars of the lowest density levels and causes the motors to turn by the numbers of rotation indicated by the input numerical values to move the unit to the position of correct focus. It is possible to perform focus adjustment of the unit with ease and high accuracy regardless of whether the unit is for forming binary or multi-valued images.