A lens array unit includes a lens array including a plurality of lenses, a first side plate, and a second side plate, the first side plate and the second side plate being configured to hold the plurality of lenses therebetween, and a frame made of resin and including a first supporting portion and a second supporting portion, the first supporting portion being in contact with an outside surface of the first side plate, the second supporting portion being in contact with an outside surface of the second side plate, the first supporting portion and the second supporting portion being configured to hold the lens array therebetween and support the lens array. The outside surface of the first side plate includes a plurality of first concave portions spaced from each other in an array direction of the lenses and configured to fit with the first supporting portion.

An image forming apparatus includes a fixing portion and an optical sensor having first and second positioning holes and a connector. The fixing portion includes first and second screw holes, a through-hole, and an extension portion. If the optical sensor is placed on the fixing portion in a first orientation where the first positioning hole is aligned with the first screw hole and the second positioning hole is aligned with the second screw hole, the connector is inserted into the through-hole without interfering with the extension portion so that screws fix the optical sensor. If the optical sensor is placed on the fixing portion in a second orientation where the first positioning hole is aligned with the second screw hole and the second positioning hole is aligned with the first screw hole, the connector interferes with the extension portion so that the connector is not inserted into the through-hole.

An image forming apparatus includes a fixing portion and an optical sensor having first and second positioning holes and a connector. The fixing portion includes first and second screw holes, a through-hole, and an extension portion. If the optical sensor is placed on the fixing portion in a first orientation where the first positioning hole is aligned with the first screw hole and the second positioning hole is aligned with the second screw hole, the connector is inserted into the through-hole without interfering with the extension portion so that screws fix the optical sensor. If the optical sensor is placed on the fixing portion in a second orientation where the first positioning hole is aligned with the second screw hole and the second positioning hole is aligned with the first screw hole, the connector interferes with the extension portion so that the connector is not inserted into the through-hole.

A lens array unit includes a lens array including a plurality of lenses, a first side plate, and a second side plate, the first side plate and the second side plate being configured to hold the plurality of lenses therebetween, and a frame made of resin and including a first supporting portion and a second supporting portion, the first supporting portion being in contact with an outside surface of the first side plate, the second supporting portion being in contact with an outside surface of the second side plate, the first supporting portion and the second supporting portion being configured to hold the lens array therebetween and support the lens array. The outside surface of the first side plate includes a plurality of first concave portions spaced from each other in an array direction of the lenses and configured to fit with the first supporting portion.