Unit sheet-stacks each as a stack of a given number of sheets are successively transferred to a bundling device for bundling the unit sheet-stacks. The bundling device includes a unit sheet-stack transfer mechanism, a mechanism for previously forming a bundling loop, and a mechanism for bundling the unit sheet-stack. The transfer mechanism receives the unit sheet-stack in a state that each sheet is postured substantially horizontally and transfers the unit sheet-stack in a state that each sheet is postured substantially vertically to a given position. The bundling loop forming mechanism previously forms a bundling loop by winding the free end portion of a tape by a given number of turns. The bundling mechanism includes a device for inserting the unit sheet-stack fed from the transfer mechanism into the bundling loop in a state that each sheet is postured substantially vertically, a device for squeezing the bundling loop around the unit sheet-stack inserted, and a device for bonding the outer portion of the squeezed bundling loop onto the peripheral portion of the bundling loop.