A sheet folding apparatus including a sheet supporting member which has an inclined surface to support a stack of sheets, a sheet position adjuster which adjusts a position of the stack of sheets along the inclined surface, a sheet pressing unit which presses the stack of sheets against the inclined surface, a folding unit which folds the stack of sheets pressed by the sheet pressing unit, and a controller which controls the sheet pressing unit to press the stack of sheets on the sheet position adjuster against the inclined surface, and to create a gap between the stack of sheets on the sheet position adjuster and the sheet pressing unit to receive a sheet onto the sheet position adjuster.