A power source system (1) includes a fuel cell (40) and a battery (20) connected in parallel. A DC-DC converter (30) is connected to the battery (20) side. A maximum output ratio of the fuel cell (40) and the battery (20) is set in a range in which the output of the fuel cell falls within 65 to 80% of the total output. Accordingly, electric power loss due to the DC-DC converter (30) is controlled, whereby a high energy efficiency can be achieved.