A detection system for vehicle comprising: a reader provided in a vehicle and configured to transmit and receive a radio signal and to transmit a transmission signal including at least a radio signal for supplying electric power; a first detector driven by the radio signal for supplying electric power included in the transmission signal when receiving the transmission signal and capable of transmitting a first response signal output in response to the transmission signal to the reader, wherein the seat includes at least a seat surface and a backrest, the first detector is disposed in the seat surface or the backrest, wherein the first response signal transmitted to the reader is blocked by sitting of a passenger on the seat, and the reader determines a sitting state of a passenger based on whether the first response signal is received in response to the transmission signal, the reader determines the sitting state of the passenger on the seat based on the first response signal, determines a seat belt wearing state based on a second response signal, and outputs, when the passenger is in the sitting state and does not wear the seat belt, a warning signal to a notifying unit, and the notifying unit warns the passenger based on the warning signal; a second detector driven by the radio signal for supplying electric power included in the transmission signal; characterized in that the second detector detects the seat belt wearing state of the seat, wherein the second detector includes a switching circuit configured to switch to an ON state or an OFF state based on the seat belt waring state of the seat, the second detector transmits the second response signal to the reader depending on the ON state or the OFF state of the switching circuit, and the second detector transmits the second response signal to the reader when the seat belt switches from a wearing state to a non-wearing state. a reader provided in a vehicle and configured to transmit and receive a radio signal and to transmit a transmission signal including at least a radio signal for supplying electric power; a first detector driven by the radio signal for supplying electric power included in the transmission signal when receiving the transmission signal and capable of transmitting a first response signal output in response to the transmission signal to the reader, wherein the seat includes at least a seat surface and a backrest, the first detector is disposed in the seat surface or the backrest, wherein the first response signal transmitted to the reader is blocked by sitting of a passenger on the seat, and the reader determines a sitting state of a passenger based on whether the first response signal is received in response to the transmission signal, the reader determines the sitting state of the passenger on the seat based on the first response signal, determines a seat belt wearing state based on a second response signal, and outputs, when the passenger is in the sitting state and does not wear the seat belt, a warning signal to a notifying unit, and the notifying unit warns the passenger based on the warning signal; a second detector driven by the radio signal for supplying electric power included in the transmission signal; characterized in that the second detector detects the seat belt wearing state of the seat, wherein the second detector includes a switching circuit configured to switch to an ON state or an OFF state based on the seat belt waring state of the seat, the second detector transmits the second response signal to the reader depending on the ON state or the OFF state of the switching circuit, and the second detector transmits the second response signal to the reader when the seat belt switches from a wearing state to a non-wearing state.