To provide an automatic tensioner suppressing the unreasonable inclination of an arm by preventing the unreasonable eccentric abrasion of bushes disposed into two-divided states.

This automatic tensioner is provided with the fixing support shaft 1, the arm 2 elastically energized in a rotating direction with its one end side rotatably and externally fitted and supported to the support shaft 1, and a tension pulley 3 rotatably supported about an axis approximately in parallel to the turning center of the arm 2 in the other end side of the arm 2. The bushes 7a and 7b as two sliding bearings are interposed in axially separated states between the outer circumferential face of the support shaft 1 and the internal circumferential face in one end portion of the arm 2. Prescribed ranges in the outer circumferential surface of one bush 27 and in the internal circumferential surface in one end portion of the arm 2 are formed into shapes gradually enlarging in the direction of separating from the other bush 7a.