To provide an automatic tensioner for preventing a friction member from being deviated in the peripheral direction to generate friction resistance uniformly with respect to the peripheral direction and damp the shock-vibration transmitted from a transmission belt efficiently.

In this automatic tensioner, each of four friction members 6 is brought into contact with a first friction face 6a and a second friction face 6b and is brought into contact with end faces in the peripheral direction of two projections 20 among four projections 20 with respect to the peripheral direction. The friction members 6 are prevented to deviate in the peripheral direction owing to these four projections 20 to generate friction resistance uniformly between the first friction face 6a and the second friction face 6b, respectively. Consequently, it is possible to damp the shock-vibration transmitted from the transmission belt efficiently.