detecting positions of the image bearing belt using {HEAD ~ a first sensor} and a second sensor disposed in {TAIL ~ a different position} from that of the first sensor; and 
detecting positions of the image bearing belt using { HEAD ~ a first sensor } and a second sensor disposed in { TAIL ~ a different position } from that of the first sensor ; and
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH