In a color laser printer, each scanner unit as an exposure scanning unit is positioned on a lateral side of a developing unit, so as not to face the surface of a photoconductor drum as a photoconductor. The scanner unit emits a laser light scanned in a direction perpendicular to a rotation axis of the photoconductor drum. A last mirror, having a reflecting surface at an angle of 45.degree. with respect to the direction of the rotation axis of the photoconductor drum, is arranged to face the surface of the photoconductor drum. The last mirror reflects the laser light emitted by the scanner unit in the direction of the rotation axis of the photoconductor drum and changes the traveling direction by 90.degree. to irradiate the surface of the photoconductor drum. This configuration allows a process cartridge including the developing unit to be attached and detached from above.