Direct-current power source apparatus

A direct-current power source apparatus in which a lithium-ion capacitor unit is used as an electric storage system and which can fully utilize the lithium-ion capacitor unit and maintain the supply of electricity to a load is provided. An electric storage system includes a lithium-ion capacitor unit and a lead-acid battery connected in parallel with a load, and a voltage detecting section that detects the voltage of the lithium-ion capacitor unit. When the voltage detecting section detects that the voltage of the lithium-ion capacitor unit has reached a unit lower-limit voltage, a control circuit outputs a conduction signal for causing a switching circuit to get into a conductive state. When the switching circuit gets into a conductive state, the secondary battery supplies an electric power to a motor. At this time, the secondary battery charges the lithium-ion capacitor unit.