A fusing device for fusing an unfused toner image formed on a recording medium, to the recording medium, comprising: a regulating member having a flat surface and an arc-shaped surface and configured to regulate a nip formed by an endless belt and a roller; a switching mechanism configured to switch the fusing device between a first mode and a second mode in which the unfused toner image is fused to the recording medium; and a side plate that is a member of a body of the fusing device, wherein 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, characterized in that the switching mechanism includes a holding member configured to rotatably hold the roller, a first biasing member configured to bias the holding member in a direction along the recording medium entry direction, and an actuation member configured to move the holding member in a direction opposite to the direction along the recording medium entry direction, the holding member holds the roller so that the roller is allowed to move in the direction along the recording medium entry direction and in the direction opposite to the direction along the recording medium entry direction, the holding member has a guide hole that is an oblong hole extending in the recording medium entry direction and provided upstream of the holding member in the recording medium entry direction, a guide pin is fixed to the side plate and fitted in the guide hole, and the holding member is held for movement along the guide hole in the recording medium entry direction that is parallel to the flat surface. a regulating member having a flat surface and an arc-shaped surface and configured to regulate a nip formed by an endless belt and a roller; a switching mechanism configured to switch the fusing device between a first mode and a second mode in which the unfused toner image is fused to the recording medium; and a side plate that is a member of a body of the fusing device, wherein 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, characterized in that the switching mechanism includes a holding member configured to rotatably hold the roller, a first biasing member configured to bias the holding member in a direction along the recording medium entry direction, and an actuation member configured to move the holding member in a direction opposite to the direction along the recording medium entry direction, a holding member configured to rotatably hold the roller, a first biasing member configured to bias the holding member in a direction along the recording medium entry direction, and an actuation member configured to move the holding member in a direction opposite to the direction along the recording medium entry direction, the holding member holds the roller so that the roller is allowed to move in the direction along the recording medium entry direction and in the direction opposite to the direction along the recording medium entry direction, the holding member has a guide hole that is an oblong hole extending in the recording medium entry direction and provided upstream of the holding member in the recording medium entry direction, a guide pin is fixed to the side plate and fitted in the guide hole, and the holding member is held for movement along the guide hole in the recording medium entry direction that is parallel to the flat surface.