A power transmission device (1, 2, 11) that transmits first AC power in a noncontact manner to a power receiving device having a first resonator, the first resonator receiving the first AC power, the power transmission device comprising: a second resonator that electromagnetically couples with the first resonator to transmit the first AC power to the first resonator; an inverter that generates the first AC power by using either a frequency f11 or a frequency f12, the frequency f11 being lower than a first resonance frequency f0 between the first resonator and the second resonator, the frequency f12 being higher than the first resonance frequency f0; an oscillator that generates the second AC power by using either a frequency f10 or a frequency f20, the frequency f10 being lower than a second resonance frequency fr between the first resonator and the second resonator, the frequency f20 being higher than the second resonance frequency fr; a foreign object detector that determines whether or not a foreign object is present between the first resonator and the second resonator based on a physical quantity in the second resonator that changes according to the second AC power of either the frequency f10 or the frequency f20; and power transmission control circuitry operative to: set a foreign object detection period in which a foreign object is detected by the foreign object detector, between i) a first power transmission period in which the first AC power is transmitted from the second resonator to the first resonator and ii) a second power transmission period subsequent to the first power transmission period; set to the inverter, one of the frequency f11 or the frequency f12 in the first power transmission period; and set to the inverter, the other of the frequency f11 or the frequency f12 in the second power transmission period if it is determined that the foreign object is present in the foreign object detection period. a second resonator that electromagnetically couples with the first resonator to transmit the first AC power to the first resonator; an inverter that generates the first AC power by using either a frequency f11 or a frequency f12, the frequency f11 being lower than a first resonance frequency f0 between the first resonator and the second resonator, the frequency f12 being higher than the first resonance frequency f0; an oscillator that generates the second AC power by using either a frequency f10 or a frequency f20, the frequency f10 being lower than a second resonance frequency fr between the first resonator and the second resonator, the frequency f20 being higher than the second resonance frequency fr; a foreign object detector that determines whether or not a foreign object is present between the first resonator and the second resonator based on a physical quantity in the second resonator that changes according to the second AC power of either the frequency f10 or the frequency f20; and power transmission control circuitry operative to: set a foreign object detection period in which a foreign object is detected by the foreign object detector, between i) a first power transmission period in which the first AC power is transmitted from the second resonator to the first resonator and ii) a second power transmission period subsequent to the first power transmission period; set to the inverter, one of the frequency f11 or the frequency f12 in the first power transmission period; and set to the inverter, the other of the frequency f11 or the frequency f12 in the second power transmission period if it is determined that the foreign object is present in the foreign object detection period. set a foreign object detection period in which a foreign object is detected by the foreign object detector, between i) a first power transmission period in which the first AC power is transmitted from the second resonator to the first resonator and ii) a second power transmission period subsequent to the first power transmission period; set to the inverter, one of the frequency f11 or the frequency f12 in the first power transmission period; and set to the inverter, the other of the frequency f11 or the frequency f12 in the second power transmission period if it is determined that the foreign object is present in the foreign object detection period.