A power transmission device includes: an oscillator including first through fourth switching devices, and converting DC power into AC power by pulses supplied thereto; a power transmission antenna delivering the AC power to a load of a power receiving device; memory holding a predetermined value indicating at least one of required voltage, current, and power; a receiving circuit receiving information of at least one of voltage, current, and power, supplied to the load; and a control circuitry changing a phase difference Δθ between pulses supplied to the first and fourth switching devices, and between pulses supplied to the second and third switching devices, thereby changing voltage of the AC power, deciding frequency of AC power where efficiency is greatest, converging the at least one of voltage, current, and power, of which information has been received, to the predetermined value range, and outputting AC power where the efficiency is the greatest.