In a fixing device, a pressure unit is pressed toward a roller by an arm, to form a nip region. The pressure unit includes a belt configured to contact the roller in a nip region, a pressure pad configured to press the belt in combination with the roller, and a side guide including an end guide surface configured to guide a side end of the belt, and a stay configured to support the pressure pad. A supporting member is located on an outer side of the side guide that is opposite to an inner side on which the end guide surface faces. The supporting member is held between the stay and the arm, the supporting member comprising a contact surface which contacts the side guide and on which the side guide is swayably supported.