A power transmission device which is connected between an AC wiring system connected to an AC power supply, and a DC power supply, and which transmits power from the DC power supply to the AC wiring system, the power transmission device comprising: an AC power supply voltage signal generator connected to the AC wiring system and configured to detect a polarity and a magnitude of voltage in the AC wiring system and to generate an AC power supply voltage signal; a power converter configured to repeatedly open and close a connection between the DC power supply and the AC wiring system in accordance with changes in the polarity and the magnitude of voltage detected by the AC power supply voltage signal generator, thereby converting a DC power outputted from the DC power supply, into an AC power; a DC voltage conversion unit including a first isolation transformer isolating an input side and an output side from each other, the DC voltage conversion unit being configured to perform a voltage conversion of DC voltage of the DC power supply and to apply resultant voltage to the power converter; a switching element configured to switch between a closed state in which DC voltage of the DC power supply is applied to the input side of the DC voltage conversion unit, and an open state in which the DC voltage is not applied; and a switch open/close signal generator configured to generate a switch open/close signal for opening or closing the switching element, wherein the switch open/close signal generator includes: a control signal generation unit configured to generate a control signal formed from a predetermined pulse signal; and a pulse width determination unit configured to receive the control signal and generate a delayed signal obtained by delaying rising of the control signal, and when the delayed signal becomes a value corresponding to a magnitude of voltage of the AC power supply voltage signal, to cause the control signal to fall, thereby making the control signal into the switch open/close signal. an AC power supply voltage signal generator connected to the AC wiring system and configured to detect a polarity and a magnitude of voltage in the AC wiring system and to generate an AC power supply voltage signal; a power converter configured to repeatedly open and close a connection between the DC power supply and the AC wiring system in accordance with changes in the polarity and the magnitude of voltage detected by the AC power supply voltage signal generator, thereby converting a DC power outputted from the DC power supply, into an AC power; a DC voltage conversion unit including a first isolation transformer isolating an input side and an output side from each other, the DC voltage conversion unit being configured to perform a voltage conversion of DC voltage of the DC power supply and to apply resultant voltage to the power converter; a switching element configured to switch between a closed state in which DC voltage of the DC power supply is applied to the input side of the DC voltage conversion unit, and an open state in which the DC voltage is not applied; and a switch open/close signal generator configured to generate a switch open/close signal for opening or closing the switching element, wherein the switch open/close signal generator includes: a control signal generation unit configured to generate a control signal formed from a predetermined pulse signal; and a pulse width determination unit configured to receive the control signal and generate a delayed signal obtained by delaying rising of the control signal, and when the delayed signal becomes a value corresponding to a magnitude of voltage of the AC power supply voltage signal, to cause the control signal to fall, thereby making the control signal into the switch open/close signal. a control signal generation unit configured to generate a control signal formed from a predetermined pulse signal; and a pulse width determination unit configured to receive the control signal and generate a delayed signal obtained by delaying rising of the control signal, and when the delayed signal becomes a value corresponding to a magnitude of voltage of the AC power supply voltage signal, to cause the control signal to fall, thereby making the control signal into the switch open/close signal.