Automatic document feeder and image forming apparatus

An automatic document feeder, comprising:

• an original feed table that has a detection hole and is configured to receive a plurality of originals;
• and
• an original detection sensor that detects the original on the original feed table, wherein
• the original detection sensor is a reflective optical sensor having a light-emitting unit and a light-receiving unit, and disposed under or inside of the original feed table so as to detect the original though the detection hole, wherein
• a portion of the detection hole is a light-emitting side portion corresponding to the light-emitting unit of the original detection sensor and a portion of the detection hole is a light-receiving side portion corresponding to the light-receiving unit of the original detection sensor, the light-emitting side portion and the light-receiving side portion having planar shapes different from each other, and
• the original feed table includes a light-shielding portion that is provided above the light-receiving unit of the original detection sensor, and has a slanted surface overhanging the light-receiving unit, wherein an area of the planar shape of the light-receiving side portion is smaller than an area of the planar shape of the light-emitting side portion.