A sheet folding device includes a folding path, a folding roller including at least two rollers that engage with each other to form a folding nip and rotate, a folding blade to be movable to an insertion location at which at least one sheet of paper on the folding path is pushed into the folding nip, and to a retreat location escaping from the folding path, a guide member to move along with the folding blade, located at at least one side of the folding blade, and to push the paper towards the folding roller when the folding blade is moved to the insertion location and an elastic member to apply elasticity to the guide member towards the folding roller.