A fusing device for thermally fusing a developer image transferred onto a recording sheet, comprising: a tubular fusing film; a heating element disposed inside the fusing film; a nip plate (130; 230) including a base portion which includes a first and a second portion (131B; 231C) located in positions upstream and downstream, respectively, with respect to a sheet conveyance direction, and a central portion (131A; 231A) disposed between the first and the second portion (131B; 232B, 231C), the base portion being disposed on an inner surface of the fusing film in such a manner that permits the fusing film to slide along the nip plate (130; 230), said nip plate (130; 230) being configured to receive radiation of heat from the heating element; a backup member configured such that the fusing film is nipped between the backup member and the central portion (131A; 231A) of the nip plate (130; 230) to form a nip portion between the fusing film and the backup member; and a stay configured to support the first and the second portion (131B; 231C), wherein each of the first and the second portion (131B; 231C) of the base portion is configured to be a flat plate extending along the recording sheet conveyance direction, characterized in that the first, the second and the central portion (131B; 232B, 231C; 131A; 231A) are disposed in such a manner that permits the fusing film to slide along the central portion (131A; 231A), the first portion (131B; 231B) and a downstream edge of the second portion (131B; 231C), the central portion (131A; 231A) is in a position shifted from those of the first and the second portion (131B; 231B, 231C) in a direction perpendicular to flat surfaces of the first the and second portion (131B; 231B, 231C), and the first and the second portion (131B; 231B, 231C) of the base portion are located outside a region of the central portion (131A; 231A) corresponding to the nip portion (N1; N2). a tubular fusing film; a heating element disposed inside the fusing film; a nip plate (130; 230) including a base portion which includes a first and a second portion (131B; 231C) located in positions upstream and downstream, respectively, with respect to a sheet conveyance direction, and a central portion (131A; 231A) disposed between the first and the second portion (131B; 232B, 231C), the base portion being disposed on an inner surface of the fusing film in such a manner that permits the fusing film to slide along the nip plate (130; 230), said nip plate (130; 230) being configured to receive radiation of heat from the heating element; a backup member configured such that the fusing film is nipped between the backup member and the central portion (131A; 231A) of the nip plate (130; 230) to form a nip portion between the fusing film and the backup member; and a stay configured to support the first and the second portion (131B; 231C), wherein each of the first and the second portion (131B; 231C) of the base portion is configured to be a flat plate extending along the recording sheet conveyance direction, characterized in that the first, the second and the central portion (131B; 232B, 231C; 131A; 231A) are disposed in such a manner that permits the fusing film to slide along the central portion (131A; 231A), the first portion (131B; 231B) and a downstream edge of the second portion (131B; 231C), the central portion (131A; 231A) is in a position shifted from those of the first and the second portion (131B; 231B, 231C) in a direction perpendicular to flat surfaces of the first the and second portion (131B; 231B, 231C), and the first and the second portion (131B; 231B, 231C) of the base portion are located outside a region of the central portion (131A; 231A) corresponding to the nip portion (N1; N2).