To certainly stack any size of sheets, and to improve alignment of sheets.

This image forming device includes: a paper delivery roller for transmitting the sheets delivered from a paper delivery device in a paper traveling direction; a paper stacking table for accommodating the stacked sheets in a paper stacking position; a first paper aligning member disposed on the downstream side of the paper traveling direction and for regulating the position of the paper traveling direction of the sheets by conducting reciprocating motions in the paper traveling direction; a second paper aligning member disposed on the upstream side of the paper traveling direction and for regulating the position of the paper traveling direction of the sheets; a third paper aligning member disposed along with the paper traveling direction and for regulating the position of a cross direction perpendicular to the paper traveling direction of the sheets by making reciprocating motions in the perpendicular direction of the paper traveling direction of the sheets. The paper delivery device includes a position detecting means of detecting the position of the first paper aligning member, the second paper aligning member and the third paper aligning member. The first paper aligning member and the third paper aligning member are constructed by being disposed independently of the paper stacking table.