To suppress generation of abnormal noises and partial abrasion of a friction plate caused by inclination of an arm of an automatic tensioner of an offset structure.

Minute inclination of an arm 2 is not prevented when a tension pulley 3 is arranged in an offset manner. In the state that the arm 2 is inclined, a frictional plate 5 is in total contact with a base end 21 and a support shaft 1 of the arm 2. It is thus possible to substantially uniformize contact surface pressure in all surfaces of the frictional plate in a practically used state, for preventing generation of stick slip phenomenon.