An object of the invention is to eliminate {HEAD ~ the disposition change} of {TAIL ~ plural beam spots} on the inner face of a drum 
An object of the invention is to eliminate { HEAD ~ the disposition change } of { TAIL ~ plural beam spots } on the inner face of a drum
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH