An image sensor unit includes: a light source including a light-emitting surface that emits light; a substantially rod-shaped light guide including a light emitter that linearizes the light from the light source and irradiates a sheet; an image sensor that receives light from the sheet and converts the light into an electric signal; a light condenser focusing the light from the sheet onto the image sensor; a circuit board on which the light source and the image sensor are mounted; and a frame accommodating the light guide, the light condenser and the circuit board. The light condenser and the light emitter of the light guide are disposed substantially parallel to each other. The center line of the light-emitting surface of the light source is disposed nearer the optical axis of the light condenser than the center line of the light emitter.