Fixing device controlling frequency of AC current caused to flow through helical coil causing electroconductive layer of rotatable member to generate heat through electromagnetic induction

A fixing device for fixing a toner image on a recording material, comprising:

• a rotatable member including an electroconductive layer;
• a helical coil provided at a hollow portion of said rotatable member, a helical axis of said helical coil extending along a generatrix direction of said rotatable member;
• a magnetic core provided inside a helically shaped portion formed by said helical coil;
• and
• a controller configured to control a frequency of an AC current caused to flow through said helical coil,
• wherein the AC current is caused to flow through said helical coil to cause the electroconductive layer to generate heat through electromagnetic induction heating to thereby to heat and fix the toner image on the recording material by heat of said rotatable member, and
• wherein said controller controls the frequency in a period in which a warm up of the fixing device is executed so that when the frequency is denoted by f and a resistance of the electroconductive layer with respect to a circumferential direction is denoted by R, f/R starting from a value larger than an predetermined value converges to the predetermined value.