A drive transmission device includes drive transmission members including a detection target gear, a detection device detecting the rotational position of the target gear and including a position detection member and a detection unit, and a conductive protection member protecting the detection unit. The detection unit and the protection member are assembled to the same member to be installed to an external apparatus. If there is a dimensional error in the protection member causing a portion of the protection member closest to the target gear to shift in position relative to the detection unit beyond a predetermined range, the protection member comes into contact with the detection unit in the assembling process, and is corrected in shape by the detection unit, with the shift in position relative to the detection unit of the portion of the protection member closest to the target gear kept within the predetermined range.