To provide a stacker mechanism capable of stacking the maximum amount of paper allowed by a stacker tray without a need for complicated control.

A stacker 10 comprises a stage 12 for loading a printed paper stack 54, a first sensor 26 and a second sensor 28 for detecting the state of stacking. The controller 40 controls the operation of the stage 12. A full stack sensor 30 is provided on the lower part of the stacker 10 for detecting the full state of the stack 54. After the full stack sensor 30 detects the state of full stack, the printed paper 52 is dropped down without lowering the stage 12. The dropping of the printed paper 52 on the stage 12 is repeated even after exceeding the position of a first sensor 26 until the predetermined number of stacking (predetermined arithmetic value) is reached. When the printed paper 52 reaches the predetermined arithmetic value, the dropping of the printed paper 52 is stopped. This enables the stacker to stack the predetermined amount of paper even after full stack is detected, thereby the maximum amount of paper allowed by the stacker tray can be stacked without a need for complicated control.