An image sensor unit includes a frame storing a linear illuminator that linearly illuminates a document, a rod lens array is used to form an image of light reflected from the document irradiated by the linear illuminator, and a printed circuit board on which a light-receiving sensor that converts light whose image has been formed by the rod lens array into an electrical signal is mounted. In the frame, a lens storage compartment, a linear illuminator storage compartment, and the linear illuminator are adjacently arranged substantially in parallel to each other in a longitudinal direction, with an inter-compartment portion formed in the frame interposed therebetween. At least one pin insertion opening is formed that extends from an inner wall of the lens storage compartment opposing the inter-compartment portion into an outside of the frame and through which a pressing pin is inserted, and in the inter-compartment portion, a face defining the lens storage compartment is formed as a vertical reference face with which a side plate of the rod lens array is brought into close contact for fixing. A notch used for applying an adhesive to the side plate of the rod lens array is disposed corresponding to the pin insertion opening and is formed to be open from the lens storage compartment to the linear illuminator storage compartment.