A fixing device includes a fixing roller that fixes a non-fixed toner image onto a sheet, a separation roller arranged downstream of the fixing roller to separate the sheet, and an endless fixing belt wound around the fixing roller and the separation roller. A pressing roller presses against the fixing roller via the fixing belt and cooperatively fixes the non-fixed toner image with the fixing roller. A guiding member is secured to the fixing device and has a guiding hole. The guiding member guides the separation roller when the separation roller approaches or withdraws from the fixing roller along the guiding hole. A belt tensioner creates a prescribed tension on the fixing belt by applying tension use biasing force to the separation roller in an opposite direction to the fixing roller. A driving force transmitting device directly transmits rotational driving force to the separation roller via an engaging section of the separation roller. The guiding hole has an arc shape coaxially formed with the driving force transmitting device.