A power distribution system for a building includes an electric current sensor for detecting a value of a current flowing from a commercial AC power source through a main line in a building and a storage battery installed in the building. When the current value detected by the electric current sensor reaches a predetermined value, a supply of an electric power to the building from the storage battery is initiated. The power distribution system further includes an overcurrent protection unit. The building includes sections individually equipped with electric power supply systems, and the overcurrent protection unit protects the main line of the building from an overcurrent by initiating the supply of the electric power from the storage battery to the building when the current value detected by the electric current sensor becomes equal to or greater than a predetermined current value for the initiation of main line protection.