An imaging unit includes a developer unit which is supported rotatably about a predetermined rotation axis thereof by means of a drum unit. Spacing regulation sleeves are respectively fitted around opposite ends of a support shaft of a developer roller for regulating a spacing between the photoreceptor drum and the developer roller. A driven gear is engaged with one end of the support shaft of the developer roller for integral rotation thereof, and adapted to be driven by a drive transmission gear. By utilizing a driving force applied to the driven gear, the developer unit is biased about the rotation axis to press one spacing regulation sleeve fitted around one end of the support shaft of the developer roller against the photoreceptor drum. The other spacing regulation sleeve fitted around the other end of the support shaft of the developer roller is resiliently pressed against the photoreceptor drum by means of a resilient member.