A fixing device comprising: an endless belt rotatable in a predetermined direction of rotation; a nip formation assembly disposed opposite an inner circumferential surface of the endless belt; and an opposed rotary body pressed against a part of the nip formation assembly via the endless belt to form a fixing nip between the endless belt and the opposed rotary body through which a recording medium bearing a toner image is conveyed, wherein the nip formation assembly includes: a base pad defining the fixing nip and including: a pressure portion pressing against the opposed rotary body via the endless belt; an extension portion contiguous to and disposed upstream from the pressure portion in a recording medium conveyance direction, the extension portion not pressing against the opposed rotary body via the endless belt and over which the endless belt slides; and a curved portion disposed upstream from the extension portion in the recording medium conveyance direction and smoothly blending into the extension portion, the curved portion not pressing against the opposed rotary body the fixing device further comprising a belt holder contacting and rotatably supporting each lateral end of the endless belt in an axial direction thereof and C-shaped in cross-section to create an opening disposed opposite the fixing nip, characterized in that a hypothetical circle overlaps an outer circumference of the C-shaped belt holder and a hypothetical extension overlaps and extends from the fixing nip in the recording medium conveyance direction, and the curved portion of the base pad of the nip formation assembly is situated at a position inside a region enclosed by the hypothetical circle and the hypothetical extension and spaced apart from the hypothetical circle and the hypothetical extension. an endless belt rotatable in a predetermined direction of rotation; a nip formation assembly disposed opposite an inner circumferential surface of the endless belt; and an opposed rotary body pressed against a part of the nip formation assembly via the endless belt to form a fixing nip between the endless belt and the opposed rotary body through which a recording medium bearing a toner image is conveyed, wherein the nip formation assembly includes: a base pad defining the fixing nip and including: a pressure portion pressing against the opposed rotary body via the endless belt; an extension portion contiguous to and disposed upstream from the pressure portion in a recording medium conveyance direction, the extension portion not pressing against the opposed rotary body via the endless belt and over which the endless belt slides; and a curved portion disposed upstream from the extension portion in the recording medium conveyance direction and smoothly blending into the extension portion, the curved portion not pressing against the opposed rotary body the fixing device further comprising a belt holder contacting and rotatably supporting each lateral end of the endless belt in an axial direction thereof and C-shaped in cross-section to create an opening disposed opposite the fixing nip, characterized in that a hypothetical circle overlaps an outer circumference of the C-shaped belt holder and a hypothetical extension overlaps and extends from the fixing nip in the recording medium conveyance direction, and the curved portion of the base pad of the nip formation assembly is situated at a position inside a region enclosed by the hypothetical circle and the hypothetical extension and spaced apart from the hypothetical circle and the hypothetical extension. a pressure portion pressing against the opposed rotary body via the endless belt; an extension portion contiguous to and disposed upstream from the pressure portion in a recording medium conveyance direction, the extension portion not pressing against the opposed rotary body via the endless belt and over which the endless belt slides; and a curved portion disposed upstream from the extension portion in the recording medium conveyance direction and smoothly blending into the extension portion, the curved portion not pressing against the opposed rotary body the fixing device further comprising a belt holder contacting and rotatably supporting each lateral end of the endless belt in an axial direction thereof and C-shaped in cross-section to create an opening disposed opposite the fixing nip, characterized in that a hypothetical circle overlaps an outer circumference of the C-shaped belt holder and a hypothetical extension overlaps and extends from the fixing nip in the recording medium conveyance direction, and the curved portion of the base pad of the nip formation assembly is situated at a position inside a region enclosed by the hypothetical circle and the hypothetical extension and spaced apart from the hypothetical circle and the hypothetical extension.