Automatic tensioner

An automatic tensioner is provided with: a base having an outer cylinder part; an arm; a pulley on which the transmission belt is wound; a friction member sandwiched between the inner peripheral surface of the outer cylindrical portion and the arm; and a coil spring that presses the friction member against the arm in the axial direction and urges the arm to rotate in the X direction with respect to the base, the friction member having: a locking surface that is positioned closer to the X direction side than the arc surface, that is locked to the arm, and that is inclined so as to be closer to the X direction side toward the outside in the radial direction; the friction member and the arm have a restricting means in which protrusions and recesses formed on the engagement surface of the friction member and protrusions and recesses formed on the engagement surface of the protruding portion of the arm engage with each other. 一种自动张紧器，具备：基座，具有外筒部；臂；带轮，供传动带卷挂；摩擦构件，夹在外筒部的内周面与臂之间；及螺旋弹簧，将摩擦构件向臂在轴向上压靠，对臂相对于基座向X方向转动施力，摩擦