A reading apparatus for reading photoelectrically an original includes a light source unit having at least one fluorescent lamp, a CCD sensor for receiving a light which has been generated from the fluorescent lamp and reflected on the original, a monitor for monitoring an output signal from the CCD sensor, and a controller for controlling the CCD output signal to be at a predetermined level by changing the electric power supplied to the fluorescent lamp, thereby maintaining the temperature of the fluorescent lamp at a predetermined degree.