A sleeve-folding assistance device for assisting a back-revealing sleeve-folding device is provided. The sleeve-folding assistance device includes a pair of right and left sleeve guide portions which disposes a body portion of a garment having the body portion and both sleeve portions on a loading conveyor to transport the garment from a front to a rear, and moves to approach both sides of the loading conveyor so as to guide both sleeve portions of the garment hung down on both sides of the loading conveyor to a sleeve-folding device for performing back-revealing sleeve-folding, in which when the pair of right and left sleeve guide portions approaches both sides of the loading conveyor and transports the body portion from the front to the rear on the loading conveyor, the pair of right and left sleeve guide portions lifts both sleeve portions of the garment, respectively.