An image forming device includes a paper supply apparatus including an adjustable knock-up plate to store a plurality of papers and having a lifter, a pick-up roller and feed roller to pick up each of the plurality of papers and feed each of the plurality of papers into the image forming device respectively, a motor to drive a plurality of units in the image forming device including the knock-up plate, and a brake unit to selectively transmit and disconnect power to the lifter. The lifting operation of the knock-up plate, to smoothly supply papers to the image forming device, is performed by receiving power from an existing arbitrary motor installed within the image forming device, and not from an exclusive motor that is solely used to drive and lift the knock-up plate.