An image reading unit includes an image sensor for converting light from a document into an electric signal, and a circuit board provided with the image sensor on one side. In the image reading unit, a heat dissipating member for controlling a temperature of the image sensor is attached to one side of the circuit board, and a shield member for reducing a noise is provided on the other side of the circuit board. The heat dissipating member and the shield member are connected by a heat conductive member. Also, the heat dissipating member is interposed between the image sensor and the circuit board.