A power conversion apparatus (1, 1a, 1b, 1c) comprising: a transformer that includes a primary winding and a secondary winding, which is magnetically coupled with the primary winding; an inverter circuit that is adapted to convert a direct current voltage supplied from a direct current power supply into an alternating current voltage and that is adapted to supply the alternating current voltage to the primary winding; a connector that includes a first end and a second end electrically connectable to at least either a commercial power system or a load; a first switching device inserted between the first end of the connector and a first end of the secondary winding; a second switching device inserted between the second end of the connector and the first end of the secondary winding; a third switching device inserted between the first end of the connector and a second end of the secondary winding; a fourth switching device inserted between the second end of the connector and the second end of the secondary winding; and a control unit that is adapted to control an amplitude of at least either an output voltage or an output current of the connector, wherein the control unit is adapted to turn on either the first switching device or the fourth switching device at a first on time in a first period, in which a voltage of the secondary winding is positive and the second switching device or the third switching device is turned on, wherein the control unit is adapted to turn on either the second switching device or the third switching device at a second on time in a second period, in which the voltage of the secondary winding is negative and the first switching device or the fourth switching device is turned on, after the first period, and wherein the control unit is adapted to control the amplitude of at least either the output voltage or the output current by shifting at least either the first on time or the second on time, wherein the control unit is adapted to keep the first switching device or the fourth switching device that is not turned on at the first on time turned on over the first period and the second period, and wherein the control unit is adapted to keep the second switching device or the third switching device that is not turned on at the second on time turned on over the first period and the second period. a transformer that includes a primary winding and a secondary winding, which is magnetically coupled with the primary winding; an inverter circuit that is adapted to convert a direct current voltage supplied from a direct current power supply into an alternating current voltage and that is adapted to supply the alternating current voltage to the primary winding; a connector that includes a first end and a second end electrically connectable to at least either a commercial power system or a load; a first switching device inserted between the first end of the connector and a first end of the secondary winding; a second switching device inserted between the second end of the connector and the first end of the secondary winding; a third switching device inserted between the first end of the connector and a second end of the secondary winding; a fourth switching device inserted between the second end of the connector and the second end of the secondary winding; and a control unit that is adapted to control an amplitude of at least either an output voltage or an output current of the connector, wherein the control unit is adapted to turn on either the first switching device or the fourth switching device at a first on time in a first period, in which a voltage of the secondary winding is positive and the second switching device or the third switching device is turned on, wherein the control unit is adapted to turn on either the second switching device or the third switching device at a second on time in a second period, in which the voltage of the secondary winding is negative and the first switching device or the fourth switching device is turned on, after the first period, and wherein the control unit is adapted to control the amplitude of at least either the output voltage or the output current by shifting at least either the first on time or the second on time, wherein the control unit is adapted to keep the first switching device or the fourth switching device that is not turned on at the first on time turned on over the first period and the second period, and wherein the control unit is adapted to keep the second switching device or the third switching device that is not turned on at the second on time turned on over the first period and the second period.