a first image bearing member;

a second image bearing member;

a first irradiation device configured to irradiate said first image bearing member with a first group of laser beams so as to form a latent image on said first image bearing member;

a second irradiation device configured to irradiate said second bearing member with a second group of laser beams so as to form a latent image on said second image bearing member;

a rotating polygonal mirror configured to scan the first group of laser beams from said first irradiation device on said first image bearing member, and configured to scan the second group of laser beams from said second irradiation device on said second image bearing member, respectively, in a main-scan direction; and

a supporting member configured to support said first irradiation device above a reference plane which is normal to a rotational axis of said rotating polygonal mirror, and configured to support said second irradiation device below the reference plane, wherein said supporting member is mounted for rotational movement around a common center between said first irradiation device and said second irradiation device,

wherein said supporting member supports said first irradiation device and said second irradiation device so that said first group of laser beams and said second group of laser beams are irradiated at an angle relative to the reference plane, and

wherein a pitch interval in a sub-scanning direction of the first group of laser beams on said first image bearing member and a pitch interval in a sub-scanning direction of the second group of laser beams on said second image bearing member are both changed at the same time by a single rotation of said supporting member around the common center.