A fixing device for fixing a developed image of developer onto a recording sheet by heating and fusing the developer includes a heating roller, a pressure roller and a driving force input element. The heating roller is configured to be heated by a heat source, such as a halogen heater. The pressure roller is disposed adjacent to the heating roller, such that a recording sheet is fed and conveyed between the pressure roller and the heating roller. The driving force input element is fixed to the pressure roller for causing the pressure roller to rotate. A diameter Dp of the pressure roller and a nip width N between the heating roller and the pressure roller in circumferential directions thereof have a relationship represented by the following expression: 0.24