A fixing device for thermally fixing a developing agent image to a sheet includes: a flexible tubular member defining an axis extending in an axial direction; a heater; a nip plate configured to be in sliding contact with the tubular member and having an engaging portion; a backup member configured to nip the flexible tubular member in cooperation with the nip plate; and a stay configured to support the nip plate and having an end portion in the axial direction. The stay includes: a support portion configured to support the nip plate; a plurality of hook portions provided at the end portion to support the nip plate; and a protruding portion protruding toward the backup member and configured to engage the engaging portion such that the nip plate is restricted from moving in a direction orthogonal to a direction in which the backup member confronts the nip plate.