A power transmission device comprising: a second resonator; a third resonator; an inverter adapted to generate first AC power and to transmit, via the second resonator, the first AC power wirelessly to a first resonator of a power receiving device; an oscillator adapted to generate second AC power which is smaller than the first AC power, and to transmit, via the third resonator, the second AC power to the first resonator; a foreign substance detector adapted to perform a series of multiple processes thereby to determine whether or not a foreign substance is present between the first resonator and the third resonator based on a physical quantity at the third resonator, the physical quantity varying depending on the second AC power; and a power transmission control circuitry adapted to: cause the inverter to perform an initial transmission of the first AC power if the foreign substance detector determines that the foreign substance is not present; cause the foreign substance detector to divide the series of multiple processes into a plurality of processes; repeat, after the initial transmission of the first AC power, a foreign substance detection period and a power transmission period alternately, wherein the foreign substance detection period is a period in which the foreign substance detector performs one of the plurality of processes, into which the series of multiple processes has been divided, and wherein the power transmission period is a period in which the inverter transmits the first AC power; and cause the foreign substance detector to determine whether or not the foreign substance is present as a result of performing all of the plurality of processes, into which the series of multiple processes has been divided; characterized in that the power transmission control circuitry is further adapted to cause the foreign substance detector to perform the series of multiple processes before the initial transmission of the first AC power; wherein the series of multiple processes includes a first determination process to measure a first physical quantity corresponding to a first type of foreign substance and determine whether or not the first type of foreign substance is present, and a second determination process to determine a second physical quantity corresponding to a second type of foreign substance and determine whether or not the second type of foreign substance is present, and the power transmission control circuitry is further adapted to cause the inverter to transmit the first AC power between the first determination process and the second determination process. a second resonator; a third resonator; an inverter adapted to generate first AC power and to transmit, via the second resonator, the first AC power wirelessly to a first resonator of a power receiving device; an oscillator adapted to generate second AC power which is smaller than the first AC power, and to transmit, via the third resonator, the second AC power to the first resonator; a foreign substance detector adapted to perform a series of multiple processes thereby to determine whether or not a foreign substance is present between the first resonator and the third resonator based on a physical quantity at the third resonator, the physical quantity varying depending on the second AC power; and a power transmission control circuitry adapted to: cause the inverter to perform an initial transmission of the first AC power if the foreign substance detector determines that the foreign substance is not present; cause the foreign substance detector to divide the series of multiple processes into a plurality of processes; repeat, after the initial transmission of the first AC power, a foreign substance detection period and a power transmission period alternately, wherein the foreign substance detection period is a period in which the foreign substance detector performs one of the plurality of processes, into which the series of multiple processes has been divided, and wherein the power transmission period is a period in which the inverter transmits the first AC power; and cause the foreign substance detector to determine whether or not the foreign substance is present as a result of performing all of the plurality of processes, into which the series of multiple processes has been divided; cause the inverter to perform an initial transmission of the first AC power if the foreign substance detector determines that the foreign substance is not present; cause the foreign substance detector to divide the series of multiple processes into a plurality of processes; repeat, after the initial transmission of the first AC power, a foreign substance detection period and a power transmission period alternately, wherein the foreign substance detection period is a period in which the foreign substance detector performs one of the plurality of processes, into which the series of multiple processes has been divided, and wherein the power transmission period is a period in which the inverter transmits the first AC power; and cause the foreign substance detector to determine whether or not the foreign substance is present as a result of performing all of the plurality of processes, into which the series of multiple processes has been divided;