A fixing device includes: a heat unit extending in an axial direction and comprising a heater, the heat unit having a central region in the axial direction and an end region outside of the central region; an opponent member to face the heat unit; and a cover positioned opposite to the opponent member with respect to the heat unit. The cover has: a central cover portion positioned to face the central region and spaced away from the central region by a reference distance; and a side cover portion positioned to face the end region and movable between a first position providing a first distance from the heat unit and a second position providing a second distance from the heat unit, the first distance being not more than the reference distance, and the second distance being greater than the reference distance.