A cross-sectional shape of an open-side cable 22a is formed into a round shape, and a cross-sectional shape of respective connecting units 52 between a pulley groove 50 of a pulley 46 and flange portions 51 is formed into a circular arc shape. Thus, it is possible to surely suppress damage of the open-side cable 22a caused by being strongly pressed to a corner as a conventional manner. Therefore, it is possible to improve durability of the open-side cable 22a, whereby it is possible to extend a maintenance cycle of a driving unit and obtain high reliability thereof.