In a fixing device, a nip plate is disposed on an inner surface of a tubular flexible fusing member which is flexibly deformable, and configured to be heated by a heating element disposed inside the fusing member. The fusing member is nipped between the nip plate and a backup member to form a nip portion. First and second portions of the nip plate which are located in positions upstream and downstream, respectively, with respect to a recording sheet conveyance direction and to be supported by a stay are each configured to be a flat plate extending along the recording sheet conveyance direction. A central portion between the first and second portions of the nip plate is in a position shifted from those of the first and second portions in a direction perpendicular to flat surfaces of the first and second portions.