An image reading apparatus includes a light source unit, an image sensor, an attenuator, an analog processor and an A/D converter. The attenuator generates a second analog signal by voltage-dividing a first analog signal by two resistors. The analog processor generates a third analog signal by amplifying the second analog signal with a predetermined gain. The A/D converter converts the third analog signal into a digital signal. A voltage-dividing ratio of the attenuator is not higher than a ratio of a maximum value of the amplitude of a signal inputtable to the analog processor to a maximum value of the amplitude of a signal having a possibility of being output from the image sensor.