A fixing device using induction heating includes an endless member having a metal layer of a conductive material; and an electromagnetic induction coil arranged close to said endless member, alternating current being supplied to said coil, thereby generating heat to heat up said endless member, said coil being wound onto a core member arranged in a width direction of said endless member, and an expansion of a folded portion of said coil being absorbed by a narrower portion of said core member, thereby maintaining to be constant the width of a gap between said coil and said endless member.