A fixing device comprises a first rotatable roller in a fixed position, a heater provided in the first rotatable roller for heating the same, and a second rotatable roller facing the first rotatable roller and capable of being in contact therewith. The second rotatable roller is supported movably by shifting members. Each shifting member separates the second rotatable roller from the first roller at normal temperature and presses the second roller against the first rotatable roller when the shifting member is deformed by heat from the first roller heated by the heater.