An automatic winder includes a package driving motor (41) a traverse driving motor (45), and a traverse width reducing section (63). The traverse driving motor reciprocates a traverse guide (11) traversing a yarn (20) being wound around a winding bobbin (22) rotated by the package driving motor. The traverse width reducing section has a reduced-width constant mode and a reduced-width changing mode. In the reduced-width constant mode, the traverse width reducing section controls the traverse driving motor so that during the accelerating winding period, traversing is performed with a constant reduced traverse width that is smaller than a target traverse width based on an initial traverse width set via a setting device. In the reduced-width changing mode, the traverse width reducing section controls the traverse driving motor so that during the accelerating winding period, traversing is performed with a traverse width that increases continuously from the reduced traverse width as a winding speed increases. (Fig. 1)