A detection system for vehicle includes a reader and a first detector. The reader is provided in a vehicle, transmits and receives radio signals, and transmits transmission signals including at least radio signals for supplying electric power. The first detector is driven by the radio signals for supplying electric power included in the transmission signals when receiving the transmission signals and can transmit first response signals output in response to the transmission signals to the reader. The first detector is disposed in a sitting area on a seat where the first response signals transmitted to the reader are blocked. The reader determines a sitting state of a passenger based on whether the first response signals are received in response to the transmission signals.