A light irradiation device includes a light source, a light guide member, and a holding member. The light guide member includes an incident surface, an exit surface, and a held surface. The incident surface has an elongated shape in a main scanning direction and faces a light emitting surface of the light source. The light guide member includes a securing structure in each of both end portions in the main scanning direction, which are out of a light guide area of the light guide member. The securing structure in one end portion is an engagement lug structure protruding to penetrate into a through-hole of the holding member, including a lug on a leading end, and engaging the holding member. The securing structure on another end portion is a fastening structure having a fastening hole penetrated with a fastening member and being secured to the holding member with the fastening member.

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.

An image sensor includes: a lens configured to focus light irradiated toward an object to be read and reflected by the object to be read; a sensor configured to receive light focused by the lens; a sensor board configured to mount thereon the sensor; a board retaining plate, having a casing attachment surface extending in the X direction and a sensor board attachment reference surface that is in contact with the +Y side of the sensor board and is formed in a side surface of the casing attachment surface, and configured to retain the sensor board; and a first casing configured to fix or retain the board retaining plate by fastening of a surface of the casing attachment surface.

A light irradiation device includes a light source, a light guide member, and a holding member. The light guide member includes an incident surface, an exit surface, and a held surface. The incident surface has an elongated shape in a main scanning direction and faces a light emitting surface of the light source. The light guide member includes a securing structure in each of both end portions in the main scanning direction, which are out of a light guide area of the light guide member. The securing structure in one end portion is an engagement lug structure protruding to penetrate into a through-hole of the holding member, including a lug on a leading end, and engaging the holding member. The securing structure on another end portion is a fastening structure having a fastening hole penetrated with a fastening member and being secured to the holding member with the fastening member.