The present invention pertains to an auto tensioner (1) to be provided in an auxiliary-driving belt system, including a friction member (6) that is sandwiched between an arm (3) and an inner circumferential surface of an outer cylindrical portion (21) of a base (2) in the radial direction of the outer cylindrical portion (21) and that has an arc-shaped surface (60) locked to the arm (2) and slidable with respect to the inner circumferential surface of the outer cylindrical portion (21), in which the friction member (6) is provided so that the arc-shaped surface (60) remains at a height equal to or higher than a horizontal plane (HP) passing through a central axis (R) when sliding with respect to the inner circumferential surface of the outer cylindrical portion (21) as the arm (3) oscillates.