A sheet supply device detachably attachable to an image forming apparatus includes: a main body; a sheet supply roller; an external output gear; a clutch mechanism; and first and second transmission mechanisms. The main body is provided with a sheet supply tray on which a sheet is mountable. The sheet supply roller is in contact with a sheet mounted on the sheet supply tray and configured to rotate to supply the sheet toward the image forming apparatus. The external output gear is configured to output a driving force to an outside. The clutch mechanism is configured to transmit and interrupt a driving force from the image forming apparatus. The first transmission mechanism is configured to transmit the driving force received through the clutch mechanism to the sheet supply roller. The second transmission mechanism is configured to transmit the driving force received through the clutch mechanism to the external output gear.