A power supply system disclosed here is connected to an electric power system through a distribution device. The power supply system includes a plurality of strings connected to the distribution device and a failure detector. The failure detector of the power supply system is configured to perform a first process of connecting at least one battery module to a main line to set a voltage detected by a string voltage detector at a voltage higher than a predetermined voltage in a state where a switch disconnects the distribution device and the main line, a second process of sending a disconnecting signal for disconnecting all the sweep modules from the main line, and a third process of determining whether the voltage detected by the string voltage detector is lower than the predetermined determination voltage or not after the second process.