The present disclosure provides a belt auto-tensioner that automatically adjusts tension of a belt for rotational force transmission between an engine and auxiliary machinery components, the belt auto-tensioner comprising: an upper cover that is provided with a pulley being in contact with the belt and has a receiving space provided so as to be spaced apart from the pulley; a tensioner body that is inserted into the receiving space, and provides friction attenuation to the upper cover when the upper cover rotates in a direction opposite to a direction of tension applying engagement by the tension of the belt; and a friction unit that is installed on an inner surface of the receiving space, and is formed so as to rub against an outer surface of the tensioner body when the upper cover rotates.