A mobile device position estimation system includes: an in-vehicle device that includes at least three in-vehicle antennas that transmits an electric wave; and a mobile device. The mobile device includes a reception intensity detection portion that detects a reception signal of the electric wave. The in-vehicle device or the mobile device includes a sensitivity error decision portion that decides a reception sensitivity error to cause multiple mobile device detection areas to be closest to a state where the multiple mobile device detection areas intersect at one point while deciding mobile device detection areas, an annular area decision portion that decides multiple annular areas where the mobile device exists, and a mobile device area estimation portion that estimates an overlapping area where the multiple annular areas overlap as a mobile device existence area.