To reduce intensity of external light entering a reading unit from an ejection port of a document conveyance path in reading a document with thickness by a document reading device of a sheet-through system without increasing the size of the device.

In a scanner unit of a sheet-through system in which a scanner upper part and a scanner lower part are moved according to a thickness of a document, so as to change a height of a document conveyance path formed in a gap between the scanner upper part and scanner lower part, an external light shielding member is arranged for closing an ejection port of the document conveyance path to shield external light.