A power source system comprising: a first power source that is mounted on a vehicle; a second power source that is mounted on the vehicle; a Direct Current to Direct Current converter that is mounted on the vehicle and is connected to the second power source, and that is configured to adjust power supplied from the second power source so as to output the adjusted power to a first path (L1) connected to the first power source; a first load that is mounted on the vehicle, includes a vehicle control device configured to perform predetermined control regarding at least one of traveling, steering, and braking of the vehicle regardless of a driving operation performed by a driver of the vehicle, and an electric actuator as a control target of the vehicle control device, and is connected to the first path so as to be supplied with power from the first power source; and a power source control device that is mounted on the vehicle, and is configured to control an operation of the Direct Current to Direct Current converter such that power is supplied to the first path from the second power source in a case where the predetermined control is performed; wherein the power source control device is configured to supply power from the second power source to the first path such that a voltage which is output from the Direct Current to Direct Current converter to the first path substantially matches a voltage of the first power source, characterized in that the power source control device is configured such that, when the predetermined control is started, the voltage at a terminal of the Direct Current to Direct Current converter is the same as the voltage at a terminal of the first power source. a first power source that is mounted on a vehicle; a second power source that is mounted on the vehicle; a Direct Current to Direct Current converter that is mounted on the vehicle and is connected to the second power source, and that is configured to adjust power supplied from the second power source so as to output the adjusted power to a first path (L1) connected to the first power source; a first load that is mounted on the vehicle, includes a vehicle control device configured to perform predetermined control regarding at least one of traveling, steering, and braking of the vehicle regardless of a driving operation performed by a driver of the vehicle, and an electric actuator as a control target of the vehicle control device, and is connected to the first path so as to be supplied with power from the first power source; and a power source control device that is mounted on the vehicle, and is configured to control an operation of the Direct Current to Direct Current converter such that power is supplied to the first path from the second power source in a case where the predetermined control is performed; wherein the power source control device is configured to supply power from the second power source to the first path such that a voltage which is output from the Direct Current to Direct Current converter to the first path substantially matches a voltage of the first power source, characterized in that the power source control device is configured such that, when the predetermined control is started, the voltage at a terminal of the Direct Current to Direct Current converter is the same as the voltage at a terminal of the first power source. <EOS>