A print-medium storing apparatus which is installed on a discharge path of a print-medium in an image forming apparatus, comprising: a plurality of storing units which are arranged with a multi-layer configuration and comprises: a main storing unit which is driven by a main driving source of the image forming apparatus, and at least two auxiliary storing units (230, 250) which are arranged in a multi-layer configuration above the main storing unit; wherein the at least two auxiliary storing units respectively comprise: a driving source which is controlled by a controller; a feeding unit which feeds the print-medium; a power transmission unit which transmits a driving force of the driving source to the feeding unit; and a tray on which the print-medium fed through the feeding unit is stored; wherein the feeding unit comprises: a feeding roller which feeds the print-medium; and a discharging roller which discharges the print-medium fed through the feeding roller to the tray; characterized in that the power transmission unit comprises: a plurality of gears (G11 to G19) which transmit the driving force of the driving source; a first power switching unit which transmits the driving force of the driving source connected thereto to the plurality of gears and the feeding unit, and prevents the driving force of the driving source of an upper-layered storing unit from being transmitted to the discharging roller comprised in said feeding unit; and a second power switching unit which transmits a driving force of the driving source of an upper-layered storing unit to the feeding roller of a lower-layered storing unit, and prevents the driving force of the lower-layered storing unit from being transmitted to the upper-layered storing unit; and said controller to control the drive of a feeding unit of a storing unit arranged between another storing unit through which the print-medium is to be discharged and the image forming apparatus, by controlling a driving source of the another storing unit through which the print-medium is to be discharged. a plurality of storing units which are arranged with a multi-layer configuration and comprises: a main storing unit which is driven by a main driving source of the image forming apparatus, and at least two auxiliary storing units (230, 250) which are arranged in a multi-layer configuration above the main storing unit; wherein the at least two auxiliary storing units respectively comprise: a driving source which is controlled by a controller; a feeding unit which feeds the print-medium; a power transmission unit which transmits a driving force of the driving source to the feeding unit; and a tray on which the print-medium fed through the feeding unit is stored; wherein the feeding unit comprises: a feeding roller which feeds the print-medium; and a discharging roller which discharges the print-medium fed through the feeding roller to the tray; characterized in that the power transmission unit comprises: a plurality of gears (G11 to G19) which transmit the driving force of the driving source; a first power switching unit which transmits the driving force of the driving source connected thereto to the plurality of gears and the feeding unit, and prevents the driving force of the driving source of an upper-layered storing unit from being transmitted to the discharging roller comprised in said feeding unit; and a second power switching unit which transmits a driving force of the driving source of an upper-layered storing unit to the feeding roller of a lower-layered storing unit, and prevents the driving force of the lower-layered storing unit from being transmitted to the upper-layered storing unit; and said controller to control the drive of a feeding unit of a storing unit arranged between another storing unit through which the print-medium is to be discharged and the image forming apparatus, by controlling a driving source of the another storing unit through which the print-medium is to be discharged.