Rotary operation mechanism, electronic apparatus and projector

A rotary operation mechanism is provided with: a gear that is rotatably provided in the apparatus main unit of an electronic apparatus that is provided with an apparatus main unit and an outer case that covers this apparatus main unit; a rotating knob for effecting the rotary operation of this gear; and an opening that is provided in the outer case and into which the rotating knob is inserted. The rotating knob is supported in the outer case in a state of engagement that allows free rotation inside the opening. The rotating knob is provided with a first rib at a position that is separated in the radial direction from the center of rotation, and the gear is provided with a second rib that forms a gap in which the first rib is interposed and that thus makes line contact with and engages with the first rib.