a sheet processing apparatus includes : an output unit that renders one side of a sheet convex and outputs the sheet , the sheet having the one side and other side ; a first folding unit that mountain - folds the one side of the sheet , which is rendered to be convex , to form a first fold in the sheet ; and a second folding unit that mountain - folds the one side of the sheet , which includes the first fold formed therein , to form a second fold in the sheet .