In a fixing portion, a heating roller and a pressure roller are provided. An angle of separation .theta. (.degree.), the temperature T.sub.HR (.degree. C.) of the surface of the heating roller, the diameter D.sub.PR (mm) of the pressure roller, and the width W.sub.NIP (mm) of a nip part are set to satisfy the following inequality: 114.ltoreq.0.55 T.sub.HR-2.36.theta.-1.59 D.sub.PR+8.62 W.sub.NIP.ltoreq.144.