A power supply unit including a power source and a plurality of loads receiving power from the power source, the power supply unit comprising: a branching circuit branching one power line connected to the power source into a plurality of branch lines for providing the plurality of loads with power source line, the branching circuit being located adjacent to the loads; a relay connector including a first terminal fitting connected to the power source, a plurality of second terminal fittings each connected to the loads, and a connector housing housing the first terminal fitting, the plurality of second terminal fittings, and the branching circuit, wherein the branching circuit is in connection with the first terminal fitting and the plurality of second terminal fittings, wherein the connector housing further houses a plurality of switching elements each disposed on a respective one of the plurality of branch lines, a voltage detector adapted for detecting a supply voltage inputted from the first terminal fitting, and an on/off controller adapted for intermittently switching on the switching elements and decreasing a duty ratio of an on-period of the switching elements as the supply voltage detected by the voltage detector rises; wherein an output of each of the plurality of switching elements is directly connected with a respective one of the plurality of second terminal fittings; and wherein the plurality of loads is housed in a plurality of cases, one of the plurality of cases including a connector having a third terminal fitting connected with the load, and wherein the relay connector is attached to the one of the plurality of cases, and the second terminal fittings and the third terminal fitting are engaged with each other. a branching circuit branching one power line connected to the power source into a plurality of branch lines for providing the plurality of loads with power source line, the branching circuit being located adjacent to the loads; a relay connector including a first terminal fitting connected to the power source, a plurality of second terminal fittings each connected to the loads, and a connector housing housing the first terminal fitting, the plurality of second terminal fittings, and the branching circuit, wherein the branching circuit is in connection with the first terminal fitting and the plurality of second terminal fittings, wherein the connector housing further houses a plurality of switching elements each disposed on a respective one of the plurality of branch lines, a voltage detector adapted for detecting a supply voltage inputted from the first terminal fitting, and an on/off controller adapted for intermittently switching on the switching elements and decreasing a duty ratio of an on-period of the switching elements as the supply voltage detected by the voltage detector rises; wherein an output of each of the plurality of switching elements is directly connected with a respective one of the plurality of second terminal fittings; and wherein the plurality of loads is housed in a plurality of cases, one of the plurality of cases including a connector having a third terminal fitting connected with the load, and wherein the relay connector is attached to the one of the plurality of cases, and the second terminal fittings and the third terminal fitting are engaged with each other.