An auto-tensioner comprising: a base having a cylindrical part; a rotating member (3, 203) supported so as to be rotatable with respect to the base; a pulley rotatably provided at the rotating member (3, 203); a friction member (6, 306, 606) sandwiched between an inner circumferential surface of the cylindrical part and the rotating member (3, 203) in a radial direction of the cylindrical part; and a coil spring (5, 405, 505) which has one end locked to the friction member (6, 306, 606) and the other end locked to the base and is disposed in a state of being compressed in an axial direction of the cylindrical part, thereby pressing the friction member (6, 306, 606) against the rotating member (3, 203) in the axial direction and rotationally biasing the rotating member (3, 203) in one direction with respect to the base through the friction member (6, 306, 606), characterized in that the friction member (6, 306, 606) has: an arcuate surface having a substantially fan-shaped cross-sectional shape orthogonal to the axial direction and capable of sliding along the inner circumferential surface of the cylindrical part, a first locking part (61, 661) opposed to the arcuate surface and 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 (3, 203), a second locking part (64, 464, 564) located further toward the outside in the radial direction than the first locking part (61, 661) and locked to the one end of the coil spring (5, 405, 505), and two side surfaces (62, 63) opposed to each other in the circumferential direction, and these two side surfaces (62, 63) are inclined with respect to the radial direction so as to face the opposite direction side to the one direction as they go toward the outside in the radial direction. a base having a cylindrical part; a rotating member (3, 203) supported so as to be rotatable with respect to the base; a pulley rotatably provided at the rotating member (3, 203); a friction member (6, 306, 606) sandwiched between an inner circumferential surface of the cylindrical part and the rotating member (3, 203) in a radial direction of the cylindrical part; and a coil spring (5, 405, 505) which has one end locked to the friction member (6, 306, 606) and the other end locked to the base and is disposed in a state of being compressed in an axial direction of the cylindrical part, thereby pressing the friction member (6, 306, 606) against the rotating member (3, 203) in the axial direction and rotationally biasing the rotating member (3, 203) in one direction with respect to the base through the friction member (6, 306, 606), characterized in that the friction member (6, 306, 606) has: an arcuate surface having a substantially fan-shaped cross-sectional shape orthogonal to the axial direction and capable of sliding along the inner circumferential surface of the cylindrical part, a first locking part (61, 661) opposed to the arcuate surface and 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 (3, 203), a second locking part (64, 464, 564) located further toward the outside in the radial direction than the first locking part (61, 661) and locked to the one end of the coil spring (5, 405, 505), and two side surfaces (62, 63) opposed to each other in the circumferential direction, and these two side surfaces (62, 63) are inclined with respect to the radial direction so as to face the opposite direction side to the one direction as they go toward the outside in the radial direction. an arcuate surface having a substantially fan-shaped cross-sectional shape orthogonal to the axial direction and capable of sliding along the inner circumferential surface of the cylindrical part, a first locking part (61, 661) opposed to the arcuate surface and 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 (3, 203), a second locking part (64, 464, 564) located further toward the outside in the radial direction than the first locking part (61, 661) and locked to the one end of the coil spring (5, 405, 505), and two side surfaces (62, 63) opposed to each other in the circumferential direction, and these two side surfaces (62, 63) are inclined with respect to the radial direction so as to face the opposite direction side to the one direction as they go toward the outside in the radial direction.