A fixing device comprising: a fixing roller including: a roller member having a shaft which extends in one direction and an elastic layer integrally formed on the shaft; and a fixing belt fitted onto the roller member, the fixing roller configured to rotate about an axis of the shaft as a rotation axis; a pressure roller configured to form a fixing nip portion between the pressure roller and the fixing roller; and a regulation member mounted at end portions of the shaft, each regulation member having an annular regulation surface which opposes a side edge of the fixing belt, the regulation member configured to regulate movement of the fixing belt in a rotation axis direction, wherein the regulation surface is inclined relative to a line perpendicular to the rotation axis as viewed on a cross-section parallel to the rotation axis, and the regulation surface is inclined from the rotation axis side such that the closer the regulation surface is to an outside, in a radial direction, of the fixing roller, the closer the regulation surface is to a center, in a length direction, of the shaft, the regulation surface is inclined such that, when θ1 represents an inclination angle of an inner region, in the radial direction, of the regulation surface relative to the line perpendicular to the rotation axis, and θ2 represents an inclination angle of an outer region, in the radial direction, of the regulation surface relative to the line perpendicular to the rotation axis, θ2>θ1 is satisfied, the regulation member is a disk-shaped member, each regulation member includes a hole into which the shaft is inserted, an annular flat surface opposing a side edge of the roller member and being parallel to the side edge of the elastic layer, the regulation surface connecting with an outer side portion, in the radial direction, of the annular flat surface, and an outer circumferential surface connecting with an outer side portion, in the radial direction, of the regulation surface so as to be parallel to the rotation axis, and the elastic layer of the roller member includes an inclined surface whose diameter gradually decreases toward the side edge of the elastic layer. a fixing roller including: a roller member having a shaft which extends in one direction and an elastic layer integrally formed on the shaft; and a fixing belt fitted onto the roller member, the fixing roller configured to rotate about an axis of the shaft as a rotation axis; a pressure roller configured to form a fixing nip portion between the pressure roller and the fixing roller; and a regulation member mounted at end portions of the shaft, each regulation member having an annular regulation surface which opposes a side edge of the fixing belt, the regulation member configured to regulate movement of the fixing belt in a rotation axis direction, wherein the regulation surface is inclined relative to a line perpendicular to the rotation axis as viewed on a cross-section parallel to the rotation axis, and the regulation surface is inclined from the rotation axis side such that the closer the regulation surface is to an outside, in a radial direction, of the fixing roller, the closer the regulation surface is to a center, in a length direction, of the shaft, the regulation surface is inclined such that, when θ1 represents an inclination angle of an inner region, in the radial direction, of the regulation surface relative to the line perpendicular to the rotation axis, and θ2 represents an inclination angle of an outer region, in the radial direction, of the regulation surface relative to the line perpendicular to the rotation axis, θ2>θ1 is satisfied, the regulation member is a disk-shaped member, each regulation member includes a hole into which the shaft is inserted, an annular flat surface opposing a side edge of the roller member and being parallel to the side edge of the elastic layer, the regulation surface connecting with an outer side portion, in the radial direction, of the annular flat surface, and an outer circumferential surface connecting with an outer side portion, in the radial direction, of the regulation surface so as to be parallel to the rotation axis, and the elastic layer of the roller member includes an inclined surface whose diameter gradually decreases toward the side edge of the elastic layer.