Patent ID: 7345695

Claim:
An image-forming device comprising: a photosensitive member and a scanning unit that scan the photosensitive member with a light beam, the scanning unit comprising: a light emitting unit that emits the light beam; a polygon mirror having a plurality of facets, each of the plurality of facets deflects the light beam to the photosensitive member; a driving unit that rotates the polygon mirror; a reference mark that revolves together with the polygon mirror, the reference mark indicating a reference position of the polygon mirror; a reference mark detecting unit that detects the reference mark; and a memory unit that saves a length of a time period from a first time to a second time for one of the plurality of facets when the polygon mirror is rotating, the first time being when the reference mark detecting unit detects the reference mark, the second time being when the light beam deflected by the one of the plurality of facets arrives at the photosensitive member; and a controller that starts modulating the light beam with image data when the time period has elapsed after the reference mark detecting unit detects the reference mark.