An output power control apparatus included in a power system, in which a secondary battery system (2) and a power generator (3) are connected in parallel with each other, controls the output power of this power system. The output power control apparatus detects the output power of the power generator (3) and controls the output voltage of the secondary battery system (2), based on a value obtained by subtracting the detected output power of the power generator (3) from an output power command for controlling the output power.