Fixing device

A fixing device comprising:

• a film-shaped cylinder configured to come into contact with a sheet and to rotate, the sheet moving in a first direction;
• a heater having a first surface and an opposing second surface, the heater including a heating element extending along a longitudinal direction of the heater that is perpendicular to the first direction, the first surface being configured to engage with an inner surface of the film-shaped cylinder;
• a contact body configured to engage with the inner surface of the film-shaped cylinder, the contact body configured to be heated (i) at a first position along the longitudinal direction at a first temperature increase rate and (ii) at a second position along the longitudinal direction at a second temperature increase rate that is lower than the first temperature increase rate;
• and
• a soaking member positioned to engage with the opposing second surface of the heater, the soaking member having a second thermal capacity per unit length at the second position along the longitudinal direction that is lower than a first thermal capacity per unit length at the first position along the longitudinal direction.