A fixing device, comprising: a roller; an endless belt having a portion facing the roller; and a heating member disposed such that the portion of the endless belt is between the heating member and the roller, the heating member extending in a width direction of the endless belt and pressing the portion of the endless belt against the roller such that a sheet can be passed in a sheet conveying direction through a nip formed between the roller and the portion of the endless belt pressed by the heating member and an image on the sheet can be fixed thereto, wherein the heating member includes a heat generating member comprising: - a first heat generating portion, a second heat generating portion and a third heat generating portion that are separated from each other and arranged along the width direction, and the first generating portion is independently operable with respect to the second and third heat generating portions, and - a boundary between the first and second heat generating portions and a boundary between the second and third generating portions, each of boundaries including at least a portion extending in a direction that crosses the sheet conveying direction, wherein the first heat generating portion is arranged at the center in the width direction, and the second heat generating portion is provided on both outer sides of the first heat generating portion in the width direction, and the third heat generating portion is provided on both outer sides of the second heat generating portion in the width direction, characterized in that the heating member further comprises a first electrically insulating substrate and a second electrically insulating substrate, the second electrically insulating substrate is provided below the heat generating member and on the first electrically insulating substrate, and an upper surface of the second electrically insulating substrate has the same shape as the lower surfaces of the first, second and third heat generating portions (361-1,361-2,361-3). a roller; an endless belt having a portion facing the roller; and a heating member disposed such that the portion of the endless belt is between the heating member and the roller, the heating member extending in a width direction of the endless belt and pressing the portion of the endless belt against the roller such that a sheet can be passed in a sheet conveying direction through a nip formed between the roller and the portion of the endless belt pressed by the heating member and an image on the sheet can be fixed thereto, wherein the heating member includes a heat generating member comprising: - a first heat generating portion, a second heat generating portion and a third heat generating portion that are separated from each other and arranged along the width direction, and the first generating portion is independently operable with respect to the second and third heat generating portions, and - a boundary between the first and second heat generating portions and a boundary between the second and third generating portions, each of boundaries including at least a portion extending in a direction that crosses the sheet conveying direction, wherein - a first heat generating portion, a second heat generating portion and a third heat generating portion that are separated from each other and arranged along the width direction, and the first generating portion is independently operable with respect to the second and third heat generating portions, and - a boundary between the first and second heat generating portions and a boundary between the second and third generating portions, each of boundaries including at least a portion extending in a direction that crosses the sheet conveying direction, wherein the first heat generating portion is arranged at the center in the width direction, and the second heat generating portion is provided on both outer sides of the first heat generating portion in the width direction, and the third heat generating portion is provided on both outer sides of the second heat generating portion in the width direction, characterized in that the heating member further comprises a first electrically insulating substrate and a second electrically insulating substrate, the second electrically insulating substrate is provided below the heat generating member and on the first electrically insulating substrate, and an upper surface of the second electrically insulating substrate has the same shape as the lower surfaces of the first, second and third heat generating portions (361-1,361-2,361-3).