To provide an auto tensioner mountable on a small engine.

The auto tensioner 1 comprises a cylindrical body 10 for rotatably supporting an outer ring 12 on the outside of an inner ring 11, a shaft part 20 inserted through the inside of the inner ring 11 and fixed to the engine, and a supporting part 30 arranged on the inside of the inner ring 11 for movably supporting the cylindrical body 10 on the shaft part 20. The cylindrical body 10 is a so-called ball bearing, and the shaft part 20 has a shaft supporting member 21 inserted through the inside of the inner ring 11 and a bolt 23 turnably inserted thereto. The supporting part 30 has a turning roller 31 turnably mounted on the shaft supporting member 21, a pair of intermediate rollers 37, 38 arranged opposing each other on both sides of a protruded portion 32 on one side thereof for approaching/separating each other, and first and second plate spring members 44, 45 for energizing them to approach each other. Turn suppressing springs 50 are provided on both sides of a protruded portion 33 on the other side of the turning roller 31 for suppressing the turning motion of the turning roller 31.