Abstract: An opening-and-closing body driving device includes an extendable unit including a spindle, a nut, a rod, a bearing, and a guide tube; and a driving unit. The guide tube includes an insertion portion, and an inner tube configured to allow movement of the nut in the axial direction and restrict rotation of the nut. A surface of the bearing facing a direction opposite to an insertion direction, in which the insertion portion is inserted, is defined as a first reference surface, and a surface of the housing facing a direction opposite to the insertion direction is defined as a second reference surface. A distance from the first reference surface of the bearing to a distal end of the inner peripheral wall is longer than a distance from the second reference surface of the housing to a distal end of the outer peripheral wall in the axial direction.