An angle adjuster comprising: a first arm equipped with a winding-and-tightening member and a second arm equipped with a rotation shaft portion, wherein, the rotation shaft portion is provided at the second arm so as to rotate together with the second arm, as seen in an axial direction of the rotation shaft portion, one end portion of the winding-and-tightening member and the other end portion of the winding-and-tightening member are arranged apart from each other and provided at the first arm, the first arm and the second arm are connected so that the second arm is rotatable relative to the first arm about the rotation shaft portion in a state in which a winding-and-tightening portion of the winding-and-tightening member is wound around an outer peripheral surface of the rotation shaft portion, a frictional force in a forward rotation direction of the second arm generated at a contact portion between the outer peripheral surface of the rotation shaft portion and an inner peripheral surface of the winding-and-tightening portion of the winding-and-tightening member by a rotational movement of the second arm in the forward rotation direction acts on the winding-and-tightening member in a loosening direction that loosens tightening of the outer peripheral surface of the rotation shaft portion to decrease a winding-and-tightening force of the winding-and-tightening member against the outer peripheral surface of the rotation shaft portion to thereby allow a rotation of the second arm in the forward rotation direction, and a frictional force in a reverse rotation direction of the second arm generated at the contact portion by a rotational movement of the second arm in the reverse rotation direction acts on the winding-and-tightening member in a winding-and-tightening direction that tightens the outer peripheral surface of the rotation shaft portion to increase the winding-and-tightening force of the winding-and-tightening member against the outer peripheral surface of the rotation shaft portion to thereby prevent a rotation of the second arm in the reverse rotation direction. a first arm equipped with a winding-and-tightening member and a second arm equipped with a rotation shaft portion, wherein, the rotation shaft portion is provided at the second arm so as to rotate together with the second arm, as seen in an axial direction of the rotation shaft portion, one end portion of the winding-and-tightening member and the other end portion of the winding-and-tightening member are arranged apart from each other and provided at the first arm, the first arm and the second arm are connected so that the second arm is rotatable relative to the first arm about the rotation shaft portion in a state in which a winding-and-tightening portion of the winding-and-tightening member is wound around an outer peripheral surface of the rotation shaft portion, a frictional force in a forward rotation direction of the second arm generated at a contact portion between the outer peripheral surface of the rotation shaft portion and an inner peripheral surface of the winding-and-tightening portion of the winding-and-tightening member by a rotational movement of the second arm in the forward rotation direction acts on the winding-and-tightening member in a loosening direction that loosens tightening of the outer peripheral surface of the rotation shaft portion to decrease a winding-and-tightening force of the winding-and-tightening member against the outer peripheral surface of the rotation shaft portion to thereby allow a rotation of the second arm in the forward rotation direction, and a frictional force in a reverse rotation direction of the second arm generated at the contact portion by a rotational movement of the second arm in the reverse rotation direction acts on the winding-and-tightening member in a winding-and-tightening direction that tightens the outer peripheral surface of the rotation shaft portion to increase the winding-and-tightening force of the winding-and-tightening member against the outer peripheral surface of the rotation shaft portion to thereby prevent a rotation of the second arm in the reverse rotation direction.