An electric bower supply system includes a power supply controlling element (62) configured to switch a plug-out power supply which supplies electric power (75) from a fuel cell (20) or a battery (21) to a house (70) and a plug-in power supply which supplies electric power from a commercial power source (75) disposed in the house (70) to a fuel cell vehicle (1a) on the basis of a vehicular power state of the fuel cell (20) and the battery (21) detected by a vehicular power state detecting element (61) and a household power state of the commercial power source (75) detected by a household power state detecting element (81) when a receptacle (10) of the fuel cell vehicle (1a) and an outlet (71) of the commercial power source (75) have been connected by a power cable (100).