A heat roller having a cylindrical sheet-like heating element having a resistance member embedded in an insulating member. The sheet-like heating element is arranged between an inner tube and an outer tube. The outer tube is formed so as to be longer than the inner tube for reducing non-uniformity in heat of the heat roller. Further, a thermal expansion coefficient of a material of the outer tube is greater than that of a material of the inner tube. Moreover, a triple-tube heat roller is provided.