To provide an image reading device capable of accurately discriminating document width even in the case of documents having approximate sizes in an image reading device provided with an automatic document feeding tray and an automatic document feeding guide.

This image reading device is provided with automatic document feeding guides 31a and 31b moved in response to the document width, a variable resistor 35 for changing a resistance value in response to the document width of the automatic document feeding guides 31a and 31b, and a main control unit 1 for discriminating the document width on the basis of the resistance value of the variable resistor 35. In the variable resistor 35, degree of a change of the resistance value in a movement section of the automatic document feeding guides 31a and 31b is set larger than that of other movement section.