A non-contact power supplying device comprising: a power-receiving resonating means set to have a predetermined resonant frequency; a power-feeding resonating means set to have a resonance frequency equal to the predetermined resonant frequency; an oscillating means configured to input an alternating-current power into the power-feeding resonating means; an impedance detecting means configured to detect impedance within a predetermined frequency range as viewed from a power-feeding side; and a frequency-variable means configured to set a frequency of the alternating-current power, wherein the oscillating means is configured to supply electric power to the power-receiving resonating means by producing a resonance between the power-receiving resonating means and the power-feeding resonating means, the impedance detecting means is configured to set the predetermined frequency range on the basis of a coupling state between the power-receiving resonating means and the power-feeding resonating means, the frequency-variable means is configured to set the frequency of the alternating-current power in accordance with a value of the impedance detected by the impedance detecting means within the predetermined frequency range, the impedance detecting means is configured to detect a phase of impedance as viewed from the power-feeding side within the predetermined frequency range, the frequency-variable means is configured to set a frequency value causing the phase of impedance to become equal to 0 within the predetermined frequency range, as the frequency of the alternating-current power, and in a case that there are a plurality of frequency values causing the phase of impedance to become equal to 0, the frequency-variable means sets a frequency value closest to the predetermined resonant frequency among the plurality of frequency values, as the frequency of the alternating-current power. a power-receiving resonating means set to have a predetermined resonant frequency; a power-feeding resonating means set to have a resonance frequency equal to the predetermined resonant frequency; an oscillating means configured to input an alternating-current power into the power-feeding resonating means; an impedance detecting means configured to detect impedance within a predetermined frequency range as viewed from a power-feeding side; and a frequency-variable means configured to set a frequency of the alternating-current power, wherein the oscillating means is configured to supply electric power to the power-receiving resonating means by producing a resonance between the power-receiving resonating means and the power-feeding resonating means, the impedance detecting means is configured to set the predetermined frequency range on the basis of a coupling state between the power-receiving resonating means and the power-feeding resonating means, the frequency-variable means is configured to set the frequency of the alternating-current power in accordance with a value of the impedance detected by the impedance detecting means within the predetermined frequency range, the impedance detecting means is configured to detect a phase of impedance as viewed from the power-feeding side within the predetermined frequency range, the frequency-variable means is configured to set a frequency value causing the phase of impedance to become equal to 0 within the predetermined frequency range, as the frequency of the alternating-current power, and in a case that there are a plurality of frequency values causing the phase of impedance to become equal to 0, the frequency-variable means sets a frequency value closest to the predetermined resonant frequency among the plurality of frequency values, as the frequency of the alternating-current power.