A belt fixing unit, an image forming apparatus, and a toner material are disclosed capable of obviating undesirable fixing belt deformation and low image quality. The belt fixing unit includes a fixing roller, a heating roller, a fixing belt, and a pressing roller in contact with the fixing roller to form a fixing nip having the fixing belt intervening there between, the fixing belt including at least a base member formed of a heat-resisting resin having a thickness of 90 .mu.m or smaller and acted upon by a tensile force of 0.49 N/mm or less. The toner is a low-melting point toner formed by subjecting a toner preparatory solution containing at least a prepolymer having a functional group containing a nitrogen atom, a polyester, a colorant and a releasing agent dispersed in an organic solvent to extension and/or crosslinking reactions in an aqueous medium.