A position estimation method to be applied to a contactless power supply system (1, 1', 1") configured to perform contactless electric power transmission of magnetic field resonance coupling between a ground-side device and a mobile body , the contactless power supply system including an alternating current magnetic field generation circuit provided in the ground-side device, a magnetic field detector provided in the mobile body, a transmission-side resonance circuit provided in the mobile body wherein a plurality of the magnetic field detectors is provided along a direction perpendicular to a traveling direction of the mobile body , and a reception-side resonance circuit provided in the ground side device, the position estimation method comprising: generating an alternating current magnetic field by the alternating current magnetic field generation circuit ; detecting the alternating current magnetic field by the magnetic field detector ; and estimating, based on an output of the magnetic field detectors , a relative positional relationship between the transmission-side resonance circuit that transmits electric power and the reception-side resonance circuit that receives the electric power from the transmission-side resonance circuit , wherein a frequency of the alternating current magnetic field is different from a resonance frequency of each of the transmission-side resonance circuit and the reception-side resonance circuit ; controlling, with a power supply controller , the contactless electric power transmission based on the estimated relative positional relationship; providing a plurality of the transmission-side resonance circuits along the direction perpendicular to the traveling direction of the mobile body ; estimating the relative positional relationship based on the transmission-side resonance circuit , of the plurality of the transmission-side resonance circuits , closest to the reception-side resonance circuit provided in the ground-side device, and switching, with the power supply controller, the transmission-side resonance circuit that transmits the electric power to the reception-side resonance circuit based on the relative positional relationship in the direction perpendicular to the traveling direction of the mobile body . generating an alternating current magnetic field by the alternating current magnetic field generation circuit ; detecting the alternating current magnetic field by the magnetic field detector ; and estimating, based on an output of the magnetic field detectors , a relative positional relationship between the transmission-side resonance circuit that transmits electric power and the reception-side resonance circuit that receives the electric power from the transmission-side resonance circuit , wherein a frequency of the alternating current magnetic field is different from a resonance frequency of each of the transmission-side resonance circuit and the reception-side resonance circuit ; controlling, with a power supply controller , the contactless electric power transmission based on the estimated relative positional relationship; providing a plurality of the transmission-side resonance circuits along the direction perpendicular to the traveling direction of the mobile body ; estimating the relative positional relationship based on the transmission-side resonance circuit , of the plurality of the transmission-side resonance circuits , closest to the reception-side resonance circuit provided in the ground-side device, and switching, with the power supply controller, the transmission-side resonance circuit that transmits the electric power to the reception-side resonance circuit based on the relative positional relationship in the direction perpendicular to the traveling direction of the mobile body . <EOS>