The present invention relates to an auto-tensioner containing a base having a cylindrical part, a rotating member supported rotatably to the base, a pulley provided at the rotating member, a friction member sandwiched between the cylindrical part and the rotating member, and a coil spring locked to the friction member and the base and disposed in a state of being compressed in an axial direction, thereby biasing the rotating member in one direction, in which the friction member has an arcuate surface capable of sliding along the cylindrical part, a first locking part located further toward the one direction side than the arcuate surface in a circumferential direction of the cylindrical part and locked to the rotating member, and a second locking part locked to one end of the coil spring.