In a portable device position estimation system, a vehicle-mounted system includes a transceiver and intercept devices. The vehicle-mounted system determines a first propagation time being a propagation time of a wireless signal from the transceiver to a portable device, based on a round trip time being a time between the transceiver transmitting a response request signal and the transceiver receiving a response signal from the portable device. The vehicle-mounted system determines a second propagation time being a propagation time of a wireless signal from the portable device to the intercept device based on the round trip time and a signal reception interval being a time between the intercept device receiving the response request signal and the intercept device receiving the response signal.