To provide an automatic document feeding device capable of detecting document size of non-fixed form size inexpensively with high precision, and to provide an image processing device.

An ADF (auto document feeder) 1 is provided with a side guide 21 sliding in the direction of document width to regulate a regulation end face of document, a pair of current carrying members 201 arranged in the direction of document width at a predetermined interval in the direction of document conveyance, a plurality of fitting projections 202 formed per a predetermined interval in the direction of document width of the current carrying member 201, and a connection member 203 having a predetermined resistance value and provided detachably in an arbitrary fitting projection 202 on a document set table 20 on which a plurality of documents are loaded on a reference end face. A conducting member 204 which comes into contact with the connection member 203 at an installation position of the connection member 203 and connects the connection member 203 with a conducting member 201 is formed in the side guide 21. The side guide 21 moves to the regulation end face of document having non-fixed form size to detect document width inexpensively with high precision.