Provided is a vehicle power source system including: a main battery configured to supply power to a drive motor of a vehicle; a fuel cell configured to supply power at least to the main battery; a main line connecting the main battery and the fuel cell; a first voltage translator placed on the main line and including a charging portion and a voltage transforming portion, the first voltage translator being configured to adjust an output voltage of the fuel cell; a relay provided between the charging portion and the fuel cell in the main line; and a sub-battery connected to an auxiliary machine power supply line via which power is supplied to an auxiliary machine of the vehicle or the fuel cell. The vehicle power source system includes a second voltage translator connected between the relay and the charging portion in the main line and configured to adjust power to be supplied from the sub-battery to the charging portion.