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