Fixing device

A fixing device comprising:

• a belt for heating a toner image on a recording material;
• a back-up member slidable on an inner surface of said belt;
• a pressing member for pressing said belt against said back-up member to form a nip in which the recording material is to be nip-conveyed;
• and
• a projection-recess portion, provided on a nip-forming surface of said back-up member, including a projection and a recess with respect to a recording material conveyance direction in the nip,
• wherein when a pressure applied when a toner temperature of the toner image reaches a softening temperature T1 (° C.) is P1 (kgf/cm2), a pressure applied when the toner temperature reaches an incipient fluidization temperature T2 (° C.) is P2 (kgf/cm2), the toner temperature when P(t)/η(t) wherein η(t) is a toner melt viscosity (Pa·s) and P(t) is a pressure (kgf/cm2) applied with the toner melt viscosity η(t) is maximum is T3 (° C.), and a pressure applied when the toner temperature reaches T3 (° C.) is P3 (kgf/cm2), the following relationships 1), 2) and 3) are satisfied:
• 0.3<P1/(P1+P2+P3), 1)
• 0.3<P2/(P1+P2+P3), and 2)
• 0.2<P3/(P1+P2+P3)<0.3. 3)