A drive transmission device includes a drive transmission member and a rotation shaft. The drive transmission member has an insertion portion. The rotation shaft includes a press-fit portion and another flat portion. The press-fit portion press-fits into the insertion portion after insertion of an axial tip of the rotation shaft into the insertion portion and includes a press-fit flat portion parallel to an axial direction of the rotation shaft. The other flat portion is at a position closer to the axial tip than the press-fit flat portion. The insertion portion has an inner-wall flat portion to contact the press-fit flat portion when the press-fit portion is press-fitted into the insertion portion. The other flat portion is parallel to the press-fit flat portion that has a distance from an axial center of the rotation shaft smaller than the press-fit flat portion.