To ensure a stable belt tension adjusting function for a long period.

This auto tensioner applies swinging resistance on a swinging member 3 by press-contacting a friction sliding part 51a of a first friction sliding member 51 on a friction surface 11 of a fixed shaft 1 by a pressing force of a first spring 52 in an early stage of use. At a point that the friction sliding part 51a of the first friction sliding member 51 is actually worn out after a certain period of use, a second spring 54 is expanded by a certain amount and a second friction sliding member 53 is press-contacted on the friction surface 11 by a joint pressing force of the first spring 52 and the second spring 54. Swinging resistance, thereby, is continuously applied on the swinging member 3.