A power source apparatus which is mounted to a vehicle with an alternator and a constant voltage control means capable of adjusting a voltage of electric power generated by the alternator to a set voltage, the power source apparatus comprising: a lead-acid battery electrically connected to the alternator; and a secondary battery, electrically connected in parallel to the lead-acid battery, having an output power density and an energy density which is higher than an output power density and an energy density of the lead-acid battery, wherein an open circuit voltage and an internal resistance of the lead-acid battery and an open circuit voltage and an internal resistance of the secondary battery are determined in order to satisfy the following conditions (a), (b), and (c): (a) an equal voltage point, at which the open circuit voltage of the lead-acid battery becomes equal to the open circuit voltage of the secondary battery, is present in a use range of state of charge of the lead-acid battery and a use range of SOC of the secondary battery; (b) the open circuit voltage of the secondary battery is higher than the open circuit voltage of the lead-acid battery at an upper limit side from the equal voltage point in the use range of SOC of the secondary battery; and (c) a terminal voltage of the secondary battery when a maximum charging current flows in the secondary battery is not more than the set voltage which is set by the constant voltage control means wherein the secondary battery supplies electric power to a first load which requires stable electric power having a constant voltage or within a predetermined voltage range, and the lead acid battery supplies electric power to a starter motor and a second load other than the first load, and the power source apparatus further comprises voltage drop suppression means configured to suppress discharge from the secondary battery to the starter motor mounted to the vehicle in order to suppress a voltage drop of the secondary battery. a lead-acid battery electrically connected to the alternator; and a secondary battery, electrically connected in parallel to the lead-acid battery, having an output power density and an energy density which is higher than an output power density and an energy density of the lead-acid battery, wherein an open circuit voltage and an internal resistance of the lead-acid battery and an open circuit voltage and an internal resistance of the secondary battery are determined in order to satisfy the following conditions (a), (b), and (c): (a) an equal voltage point, at which the open circuit voltage of the lead-acid battery becomes equal to the open circuit voltage of the secondary battery, is present in a use range of state of charge of the lead-acid battery and a use range of SOC of the secondary battery; (b) the open circuit voltage of the secondary battery is higher than the open circuit voltage of the lead-acid battery at an upper limit side from the equal voltage point in the use range of SOC of the secondary battery; and (c) a terminal voltage of the secondary battery when a maximum charging current flows in the secondary battery is not more than the set voltage which is set by the constant voltage control means (a) an equal voltage point, at which the open circuit voltage of the lead-acid battery becomes equal to the open circuit voltage of the secondary battery, is present in a use range of state of charge of the lead-acid battery and a use range of SOC of the secondary battery; (b) the open circuit voltage of the secondary battery is higher than the open circuit voltage of the lead-acid battery at an upper limit side from the equal voltage point in the use range of SOC of the secondary battery; and (c) a terminal voltage of the secondary battery when a maximum charging current flows in the secondary battery is not more than the set voltage which is set by the constant voltage control means wherein the secondary battery supplies electric power to a first load which requires stable electric power having a constant voltage or within a predetermined voltage range, and the lead acid battery supplies electric power to a starter motor and a second load other than the first load, and the power source apparatus further comprises voltage drop suppression means configured to suppress discharge from the secondary battery to the starter motor mounted to the vehicle in order to suppress a voltage drop of the secondary battery.