The present invention provides a fixing device that may include a nip member, a rotating body and an endless belt. The endless belt may be in sliding contact with the nip member. The rotating body may constitute a nip region. The nip region may define a first, second and third imaginary planes. The first imaginary plane may be perpendicular to the sliding direction. The second imaginary plane may be perpendicular to the axial direction of the rotating body and may contain a first end portion of the nip region. The third imaginary plane may contain a second end portion of the nip region. The first surface may be a convex shape in a first cross-section along the first imaginary plane, and may be concave shape concaved in a direction opposite to the rotating body in second and third cross-sections along the second and third imaginary planes.