A pulley rotatingly supporting device, comprising a three−point contact type or a four−point contact type radial ball bearing (14b), wherein an offset amount (&dgr;) as an axial distance between the center portion (&agr;) of a radial load applied from an endless belt (11) to a driven pulley (4b) and the center position (&bgr;) of the radial ball bearing (14b) is set to 40% or less of the pitch circle diameter of the radial ball bearing (14b), and the radial clearance of the radial ball bearing (14b) is set to 0.2% or less of the pitch circle diameter, whereby the durability of the endless belt (11) applied onto the driven pulley (4b) and the pulley rotatingly supporting device for supporting the driven pulley (4b) can be assured with a downsized structure.
