Fixing device

A fixing device comprising:

• an endless and rotatable belt of which an inner peripheral surface is applied with lubricant;
• a heater configured to heat the inner peripheral surface of the belt;
• a rotatable member configured to rotatably contact the belt;
• and
• a nip portion forming member in contact with the inner peripheral surface of the belt, configured to nip the belt with the rotatable member and form a nip portion,
• wherein the belt applies heat and pressure to a recording material on which a toner image is carried in the nip portion with the rotatable member to fix,
• wherein the nip portion forming member includes a holding portion, an upstream side sliding portion and a downstream side sliding portion,
• wherein the holding portion holds the heater in a rotational axis direction of the rotatable member,
• wherein the downstream side sliding portion has a first sliding surface sliding with the inner peripheral surface of the belt in a downstream of the holding portion with respect to a rotational direction of the belt,
• wherein the upstream side sliding portion has a second sliding surface sliding with the inner peripheral surface of the belt in an upstream of the holding portion with respect to the rotational direction of the belt,
• wherein the first sliding surface and the second sliding surface slide with the belt in a side of the rotatable member to the heater,
• wherein the second sliding surface has a curved shape, and when a force applied to the belt with respect to a length of the second sliding surface in the rotational axis direction is defined as w (N/mm) and a curvature radius of the second sliding surface is defined as R (mm), R/w of the second sliding surface is equal to 5 (mm2/N) or larger and equal to 80 (mm2/N) or less.