An auto tensioner to be provided in an auxiliary-driving belt system, comprising: a base having a cylindrical portion; an arm supported on the base so as to be rotatable about a center axis of an inner circumferential surface of the cylindrical portion; a tensioner pulley provided on the arm so as to be rotatable and configured to be wound with a belt of the auxiliary-driving belt-system; a friction member locked on the arm while being sandwiched between the inner circumferential surface of the cylindrical portion and the arm in a radial direction of the cylindrical portion, the friction member having a substantially fan shape in a cross-section orthogonal to the center axis and having an arc-shaped surface configured to be slidable on the inner circumferential surface of the cylindrical portion; and a coil spring configured to rotationally bias the arm in one direction with respect to the base, wherein when the arm oscillates due to a fluctuation of tension of the belt wound around the tensioner pulley, the oscillation of the arm is damped by generating a frictional force between the arc-shaped surface and the inner circumferential surface of the cylindrical portion, and wherein the friction member is configured and positioned so that the arc-shaped surface remains at a height equal to or higher than a horizontal plane passing through the center axis when it slides on the inner circumferential surface of the cylindrical portion as the arm oscillates due to a fluctuation of tension of the belt, wherein the arc-shaped surface has a center angle of smaller than 150° when viewed in a direction of the center axis, and the friction member is further configured and positioned so that the uppermost position of the inner circumferential surface of the cylindrical portion is brought into contact with an area that constitutes a central portion of the arc-shaped surface and whose center angle is 10° when viewed in the direction of the center axis, at a point in time when a predetermined initial tension is applied on the belt. a base having a cylindrical portion; an arm supported on the base so as to be rotatable about a center axis of an inner circumferential surface of the cylindrical portion; a tensioner pulley provided on the arm so as to be rotatable and configured to be wound with a belt of the auxiliary-driving belt-system; a friction member locked on the arm while being sandwiched between the inner circumferential surface of the cylindrical portion and the arm in a radial direction of the cylindrical portion, the friction member having a substantially fan shape in a cross-section orthogonal to the center axis and having an arc-shaped surface configured to be slidable on the inner circumferential surface of the cylindrical portion; and a coil spring configured to rotationally bias the arm in one direction with respect to the base, wherein when the arm oscillates due to a fluctuation of tension of the belt wound around the tensioner pulley, the oscillation of the arm is damped by generating a frictional force between the arc-shaped surface and the inner circumferential surface of the cylindrical portion, and wherein the friction member is configured and positioned so that the arc-shaped surface remains at a height equal to or higher than a horizontal plane passing through the center axis when it slides on the inner circumferential surface of the cylindrical portion as the arm oscillates due to a fluctuation of tension of the belt, wherein the arc-shaped surface has a center angle of smaller than 150° when viewed in a direction of the center axis, and the friction member is further configured and positioned so that the uppermost position of the inner circumferential surface of the cylindrical portion is brought into contact with an area that constitutes a central portion of the arc-shaped surface and whose center angle is 10° when viewed in the direction of the center axis, at a point in time when a predetermined initial tension is applied on the belt.