A vehicle detection system includes an RFID reader, a detection device, and an ECU. The RFID reader transmits a transmission signal including at least a signal that supplies power. The detection device includes a second antenna unit that mutually transmits and receives a signal to and from the RFID reader, an RFID detection circuit that is activated using the signal that supplies power included in the transmission signal received by the second antenna unit as driving power and that outputs a detection signal to the second antenna unit, and a switch circuit that switches an electrical connection portion between the second antenna unit and the RFID detection circuit to a contact state or to a non-contact state according to a state inside the vehicle. The ECU determines the state inside the vehicle on the basis of the detection signal received by the RFID reader.