An object of the invention is to reproduce a faithful image irrespective of thermal expansion of a light-scanning unit and to reproduce a faithful image without enhancing the strength of the light-scanning unit excessively. A pair of supporting portions of a light-scanning unit, each having a flat face, are supported in linear contact with a right column-shaped supporting member provided with the apparatus body. The light-scanning unit is provided on the supporting member near to a starting position where writing image data on a photosensitive body with a scanning line is started in a main scanning direction so as not to be mutually displaced relative to the supporting member. Thereby, the light-scanning unit is allowed to extend in the longitudinal direction of the supporting member due to thermal expansion of the light-scanning unit. Also, an elongated supporting member made of stainless steel penetrates a base body of a light-scanning unit made of synthetic resin which scans in scanning line of laser light on the photosensitive body to exposure, to support the light-scanning unit. The center of gravity of the light-scanning unit is positioned near to the supporting member and vertically below the supporting member, to make the moment acting on the light-scanning unit small and hold the light-scanning unit in a stable state. The supporting member suppresses deformation of the base body due to thermal expansion.