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.