Provided are a relay device and a power supply device that can switch on/off a current path between power storage units, and can suppress, if an earth fault has occurred on either power storage unit side, a voltage reduction on the other power storage unit side. A relay device includes: a serial structural portion in which a resistance unit and a switch unit are connected in series to each other; a relay unit that serves as a path for a current to flow between a first power storage unit and a second power storage unit when the switch unit is in the ON state; and a control unit configured to perform control of switching the switch unit of the relay unit.