A power supply system (120) according to an embodiment of the present invention includes a first power supply device (121), a second power supply device (122), and an addition circuit (123). The first power supply device (121) includes a first driving circuit that outputs a first direct current from a first power supply input and a first control circuit that controls the first driving circuit. The second power supply device (122) includes a second driving circuit that outputs a second direct current from a second power supply input and a second control circuit that monitors the first driving circuit and controls the second driving circuit. The addition circuit (123) is connected to an output terminal of each of the first and second power supply devices (121, 122) and outputs a third direct current that is a value obtained by adding the first and second direct currents to a load device (on-board illumination device (110)).