Patent ID: 8144381

Claim:
An image forming apparatus that performs multi-color image formation, comprising: a plurality of scanning type optical devices which respectively deflect and scan an optical beam using a rotating polygonal mirror; a detecting unit which detects a phase of rotating speed unevenness of said rotating polygonal mirror respectively provided at said plurality of scanning type optical devices; and an adjusting unit which adjusts, based on a phase of rotating speed unevenness detected for said each rotating polygonal mirror by said detecting unit, the rotating speed of each rotating polygonal mirror to reduce the phase differences between the phases, wherein said detecting unit includes: a sensing unit which outputs a sensing signal every time an optical beam reflected by each of a plurality of mirror surfaces provided at said rotating polygonal mirrors is sensed; a specifying unit which specifies a time difference between two adjacent sensing signals outputted from said sensing unit; and a determining unit which determines a frequency of the rotating speed unevenness from a plurality of time differences specified by said specifying unit and determines a phase from the determined frequency.