A document size sensor is disposed on a document feed path at a location between a document tray and a juncture and on the front side in the primary scanning direction perpendicular to a document feed direction. The document size sensor is located between a sheet edge position for B4-size document sheet and a sheet edge position for A 4 -size document sheet, which are situated on the front side in the primary scanning direction. The document size sensor detects A3 - and B 4 -size document sheets but does not detect A 4 - or B5-size document sheet. Based on a detection signal from the document size sensor, judgment is made as to whether a document sheet fed from the document tray is a larger-sized document sheet or a smaller-sized document sheet.