A fixing device of an image forming apparatus includes a heating roller, a pressing roller forming a first fixing nip by contacting the heating roller, and a pressing member disposed at a lower stream of the heating roller and forming a second fixing nip by contacting the heating roller. A fixing belt runs along a caterpillar track, and winds around the pressing roller and the pressing member. The pressing member includes a protrusion part for pressing the paper passed through the second fixing nip by greater pressure at a nip outlet than at the second fixing nip. Accordingly, as the width of the fixing nip is increased, the staying time at the fixing nip increases, thereby improving printing quality according to performance of high-speed printing. Additionally, wrap jams of the paper may be substantially prevented from occurring at the nip outlet.