Provided are a cable-driving apparatus that performs the function of an equalizer using a planet gear assembly, and a parking brake system using the cable-driving apparatus. The cable-driving apparatus includes a driving motor; a first planet gear assembly for reducing a rotational velocity of the driving motor; and a second planet gear assembly coupled to the first planet gear assembly, wherein a first parking brake cable is connected to a circumferential outer surface of a ring gear of the second planet gear assembly, and a second parking brake cable is connected to a circumferential outer surface of a carrier of the second planet gear assembly in an opposite direction to the first parking brake cable.