A system interconnection device for a decentralized power supply, comprising: a power generation device configured to generate a direct-current power; an alternating-current system power supply; a power converter being positioned between the power generation device and the system power supply, the power converter configured to convert the direct-current power generated by the power generation device into an alternating-current power and to output the alternating-current power to the system power supply; an electric circuit (21, 22) configured to connect the power converter and the system power supply; a switch (15, 15a, 15b) being provided at the electric circuit (21, 22), the switch (15, 15a, 15b) configured to be switchable to an open state where the electric circuit (21, 22) is disconnected such that an interconnection of the power converter and the system power supply is released, and to be switchable to a closed state where the electric circuit (21, 22) is conductive such that the alternating-current power from the power converter is output to the system power supply; a switch driving circuit configured to switch the switch (15, 15a, 15b) to the open state and to the closed state; a control unit including an arithmetic device, the control unit configured to generate an opening command for making the switch (15, 15a, 15b) be in the open state by the arithmetic device, to generate a closing command for making the switch (15, 15a, 15b) be in the closed state by the arithmetic device, and to output the opening command or the closing command to the switch driving circuit by the arithmetic device; and an external monitoring device being separately provided outside the arithmetic device of the control unit, characterized in that the external monitoring device is configured to monitor whether an abnormality of the arithmetic device occurs, when detecting the abnormality of the arithmetic device of the control unit, the external monitoring device is configured to generate a first opening command for making the switch (15, 15a, 15b) be in the open state, to output the first opening command to the switch driving circuit, and to restart the arithmetic device of the control unit, the restarted arithmetic device of the control unit is configured to generate a second opening command for making the switch (15, 15a, 15b) be in the open state, and to output the second opening command to the switch driving circuit, and the switch driving circuit is configured to make the switch (15, 15a, 15b) be in the open state while receiving at least one of the first opening command and the second opening command. a power generation device configured to generate a direct-current power; an alternating-current system power supply; a power converter being positioned between the power generation device and the system power supply, the power converter configured to convert the direct-current power generated by the power generation device into an alternating-current power and to output the alternating-current power to the system power supply; an electric circuit (21, 22) configured to connect the power converter and the system power supply; a switch (15, 15a, 15b) being provided at the electric circuit (21, 22), the switch (15, 15a, 15b) configured to be switchable to an open state where the electric circuit (21, 22) is disconnected such that an interconnection of the power converter and the system power supply is released, and to be switchable to a closed state where the electric circuit (21, 22) is conductive such that the alternating-current power from the power converter is output to the system power supply; a switch driving circuit configured to switch the switch (15, 15a, 15b) to the open state and to the closed state; a control unit including an arithmetic device, the control unit configured to generate an opening command for making the switch (15, 15a, 15b) be in the open state by the arithmetic device, to generate a closing command for making the switch (15, 15a, 15b) be in the closed state by the arithmetic device, and to output the opening command or the closing command to the switch driving circuit by the arithmetic device; and an external monitoring device being separately provided outside the arithmetic device of the control unit, characterized in that the external monitoring device is configured to monitor whether an abnormality of the arithmetic device occurs, when detecting the abnormality of the arithmetic device of the control unit, the external monitoring device is configured to generate a first opening command for making the switch (15, 15a, 15b) be in the open state, to output the first opening command to the switch driving circuit, and to restart the arithmetic device of the control unit, the restarted arithmetic device of the control unit is configured to generate a second opening command for making the switch (15, 15a, 15b) be in the open state, and to output the second opening command to the switch driving circuit, and the switch driving circuit is configured to make the switch (15, 15a, 15b) be in the open state while receiving at least one of the first opening command and the second opening command.