A respiratory detection device detects a behavior of breathing of an occupant on a seat of a vehicle. A seatbelt is equipped by the occupant to retain the occupant at the seat. A detection belt is mounted to the vehicle or the seat at one-end side. The detection belt is further connected to the seatbelt at the other-end side. The detection belt is equipped by the occupant together with the seatbelt when the occupant equips the seatbelt. A sensor is connected to the detection belt to detect the behavior of breathing. The detection belt retains the sensor at a position of an abdomen of the occupant when the occupant equips the seatbelt.
