To provide a sheet transfer device and an image forming device capable of reducing noises at the time when the loop of a sheet disappears.

When an inverting upper roller 53 transfers a sheet S together with a fastening outward paper discharge roller 91, the sheet transfer speed of the inverting upper roller 53 is set slower than the sheet transfer speed of the fastening outward paper discharge roller 91 till the inverting upper roller 53 transfers a predetermined quantity of sheets, so that a loop is formed in the sheet. After the inverting upper roller 53 transfers the predetermined quantity of sheets, drive means is controlled to switch acceleration while increasing the sheet transfer speed of the inverting upper roller 53 from a first sheet transfer speed to a second sheet transfer speed higher than the sheet transfer speed of the fastening outward paper discharge roller 91, and to dissolve the loop while increasing the sheet transfer speed of the inverting upper roller 53 at a lower one of accelerations before and after the switching.