To improve overall fold quality in a folding part of a sheet or a sheet bundle while securing productivity in sheet folding processing.

The sheet folding device successively receives a sheet or a sheet bundle to be folded, and folds the sheet or the sheet bundle using a fifth folding roller 205 and a sixth folding roller 206. The sheet folding device includes an additional sheet folding part 600 which is located on the downstream side in the sheet conveyance direction of the fifth folding roller 205 and the sixth folding roller 206, temporarily stacks the sheet or the sheet bundle folded by the fifth folding roller 205 and the sixth folding roller 206, is moved along the fold in the direction intersecting the conveyance direction of the sheet on the fold of the stacked sheet or sheet bundle and presses the fold again.