An image reader is disclosed which is provided with a screen for an image recorded on a microfilm to be projected thereon and a photoelectric transferring element for transforming the image into electric signals. In a light path originating in a light source and reaching the photoelectric transferring element, a beam attenuating filter for attenuating the luminous energy of the beam of light exposed on the photoelectric transferring element is disposed and used for decreasing the difference in the illuminance of the light source during the reader mode for causing the image to be projected on the screen and during the transforming mode for causing the image to be exposed on the photoelectric transferring element. The illuminance of the light source is controlled by the variation of the intermittent frequency of the voltage applied to the light source. While the operation is proceeding in the transferring mode, the illuminance of the light source is controlled with the aforementioned intermittent frequency falling within the range of sonic frequency so as to permit variation of the illuminance of the light source in a multiplicity of stages. While the operation is in this mode, the sound of the movable components of the device being operated for scanning cancels the noise generated within the range of sonic frequency.