A mobile device position estimation system includes: a vehicular system having a vehicle side transmitter transmitting an impulse signal, a vehicle side receiver receiving a response signal, and a vehicle side control unit outputting a transmission command signal of the impulse signal and receiving a notification signal of the response signal; and a mobile device having a mobile device side receiver receiving the impulse signal and a mobile device side transmitter transmitting the response signal. The vehicle side control unit includes: a reception detection unit detecting the response signal; a round-trip timer measuring a round-trip time; and a position estimation unit estimating a position of the mobile device. The position estimation unit: calculates a round-trip distance; and determines whether the mobile device is within a detection area.