A power supply system that is connected to an electric power system through a distribution device, the power supply system comprising: a plurality of strings connected to the distribution device; and a failure detector, wherein each of the strings includes a main line connected to the distribution device, a plurality of sweep modules connected to the main line in series, a switch that switches a connection state between the main line and the distribution device between connection and disconnection, a string capacitor attached to the main line in parallel with the plurality of sweep modules, and a string voltage detector attached to the main line in parallel with the plurality of sweep modules, each of the plurality of sweep modules includes a battery module, an input/output circuit configured to connect the battery modules to the main line, a first switching device disposed in series with respect to the main line and in parallel with respect to the battery module, and a second switching device disposed in a portion of the input/output circuit where the battery module is connected to the main line in series, and the failure detector is configured to perform a first process of connecting at least one of the battery modules in the plurality of sweep modules to the main line to set a voltage detected by the string voltage detector at a voltage higher than a predetermined voltage in a state where the switch disconnects the distribution device and the main line, a second process of turning all the first switching devices in the plurality of sweep modules on and all the second switching devices in the plurality of sweep modules off, and a third process of determining whether a voltage detected by the string voltage detector is lower than a predetermined determination voltage or not after the second process. a plurality of strings connected to the distribution device; and a failure detector, wherein each of the strings includes a main line connected to the distribution device, a plurality of sweep modules connected to the main line in series, a switch that switches a connection state between the main line and the distribution device between connection and disconnection, a string capacitor attached to the main line in parallel with the plurality of sweep modules, and a string voltage detector attached to the main line in parallel with the plurality of sweep modules, a main line connected to the distribution device, a plurality of sweep modules connected to the main line in series, a switch that switches a connection state between the main line and the distribution device between connection and disconnection, a string capacitor attached to the main line in parallel with the plurality of sweep modules, and a string voltage detector attached to the main line in parallel with the plurality of sweep modules, each of the plurality of sweep modules includes a battery module, an input/output circuit configured to connect the battery modules to the main line, a first switching device disposed in series with respect to the main line and in parallel with respect to the battery module, and a second switching device disposed in a portion of the input/output circuit where the battery module is connected to the main line in series, and a battery module, an input/output circuit configured to connect the battery modules to the main line, a first switching device disposed in series with respect to the main line and in parallel with respect to the battery module, and a second switching device disposed in a portion of the input/output circuit where the battery module is connected to the main line in series, and the failure detector is configured to perform a first process of connecting at least one of the battery modules in the plurality of sweep modules to the main line to set a voltage detected by the string voltage detector at a voltage higher than a predetermined voltage in a state where the switch disconnects the distribution device and the main line, a second process of turning all the first switching devices in the plurality of sweep modules on and all the second switching devices in the plurality of sweep modules off, and a third process of determining whether a voltage detected by the string voltage detector is lower than a predetermined determination voltage or not after the second process. a first process of connecting at least one of the battery modules in the plurality of sweep modules to the main line to set a voltage detected by the string voltage detector at a voltage higher than a predetermined voltage in a state where the switch disconnects the distribution device and the main line, a second process of turning all the first switching devices in the plurality of sweep modules on and all the second switching devices in the plurality of sweep modules off, and a third process of determining whether a voltage detected by the string voltage detector is lower than a predetermined determination voltage or not after the second process.