A foreign substance detection device for a card reader includes an emitting module (1), a receiving module (2) and a control processing module (3). The emitting module (1) is embedded on a wall surface of a card reader insertion opening (101) for emitting detection waves to a detection area (103) in front of the card reader insertion opening (101). The receiving module (2) is embedded on the wall surface of the card reader insertion opening (101) for receiving the detection waves emitted by the emitted the emitting module (1). The control processing module (3) identifies whether the receiving module (2) receives the detection wave reflected by the foreign substance in the detection area (103), and determines whether the foreign substance exists in the detection area (103) according to the determination result. A card reader socket which includes a case (100) and the foreign substance detection device is also provided.