A non-contact power supply apparatus, comprising: an inverter circuit which includes a plurality of conversion switching elements (Q1-Q4) electrically connected between a pair of input points (211, 212) and a pair of output points (213, 214) and which is configured to convert a direct-current voltage applied to the pair of input points (211, 212) into an alternating-current voltage by switching the plurality of conversion switching elements (Q1-Q4) and to output the alternating-current voltage from the pair of output points (213, 214); a primary coil electrically connected between the output points (213, 214) in the pair and configured to supply output power to a secondary coil in a non-contact manner when the alternating-current voltage is applied to the primary coil; a variable capacitance circuit electrically connected between the pair of output points (213, 214) and the primary coil, including an adjustment capacitor and a plurality of adjustment switching elements (Q5-Q8), and configured to adjust a magnitude of a capacity component between the pair of output points (213, 214) and the primary coil by switching the plurality of adjustment switching elements (Q5-Q8); and a control circuit configured to control the plurality of conversion switching elements (Q1-Q4) by a first drive signal (G1-G4) and to control the plurality of adjustment switching elements (Q5-Q8) by a second drive signal (G5-G8), wherein the control circuit is configured to adjust a phase difference (θ) which is a delay of a phase of the second drive signal (G5-G8) to a phase of the first drive signal (G1-G4) to a set value within a prescribed range to adjust a magnitude of the output power, and the control circuit includes an estimator configured to, while the variable capacitance circuit is in operation, estimate whether an initial mode which is an operation mode of the inverter circuit in a case of the variable capacitance circuit being disabled is a leading phase mode or a lagging phase mode, and a setter configured to set the prescribed range in accordance with the initial mode estimated by the estimator, characterized in that: the setter is configured to set the prescribed range to a range of 90 degrees to 180 degrees when the initial mode estimated by the estimator is the leading phase mode and to set the prescribed range to a range including both the range of 90 degrees to 180 degrees and a range of 270 degrees to 360 degrees when the initial mode estimated by the estimator is the lagging phase mode. an inverter circuit which includes a plurality of conversion switching elements (Q1-Q4) electrically connected between a pair of input points (211, 212) and a pair of output points (213, 214) and which is configured to convert a direct-current voltage applied to the pair of input points (211, 212) into an alternating-current voltage by switching the plurality of conversion switching elements (Q1-Q4) and to output the alternating-current voltage from the pair of output points (213, 214); a primary coil electrically connected between the output points (213, 214) in the pair and configured to supply output power to a secondary coil in a non-contact manner when the alternating-current voltage is applied to the primary coil; a variable capacitance circuit electrically connected between the pair of output points (213, 214) and the primary coil, including an adjustment capacitor and a plurality of adjustment switching elements (Q5-Q8), and configured to adjust a magnitude of a capacity component between the pair of output points (213, 214) and the primary coil by switching the plurality of adjustment switching elements (Q5-Q8); and a control circuit configured to control the plurality of conversion switching elements (Q1-Q4) by a first drive signal (G1-G4) and to control the plurality of adjustment switching elements (Q5-Q8) by a second drive signal (G5-G8), wherein the control circuit is configured to adjust a phase difference (θ) which is a delay of a phase of the second drive signal (G5-G8) to a phase of the first drive signal (G1-G4) to a set value within a prescribed range to adjust a magnitude of the output power, and the control circuit includes an estimator configured to, while the variable capacitance circuit is in operation, estimate whether an initial mode which is an operation mode of the inverter circuit in a case of the variable capacitance circuit being disabled is a leading phase mode or a lagging phase mode, and a setter configured to set the prescribed range in accordance with the initial mode estimated by the estimator, an estimator configured to, while the variable capacitance circuit is in operation, estimate whether an initial mode which is an operation mode of the inverter circuit in a case of the variable capacitance circuit being disabled is a leading phase mode or a lagging phase mode, and a setter configured to set the prescribed range in accordance with the initial mode estimated by the estimator, characterized in that: the setter is configured to set the prescribed range to a range of 90 degrees to 180 degrees when the initial mode estimated by the estimator is the leading phase mode and to set the prescribed range to a range including both the range of 90 degrees to 180 degrees and a range of 270 degrees to 360 degrees when the initial mode estimated by the estimator is the lagging phase mode.