A fusing device includes a regulating member and a switching mechanism. The regulating member has a flat surface and an arc-shaped surface and regulates a nip formed by a belt and a roller. The switching mechanism switches the fusing device between a first mode and a second mode in which an unfused toner image is fused to a recording medium. The flat surface is provided along a direction in which the recording medium enters the nip. The arc-shaped surface is provided downstream of the flat surface in the recording medium entry direction, is contiguous to the flat surface, and is curved toward the roller. In the first mode, the nip is regulated by the flat surface and the arc-shaped surface. In the second mode, the nip is regulated by the flat surface of the flat and arc-shaped surfaces.