A heating device for use in an induction heating type fixing system is composed of a sleeve 12 made of an electrical conductive material and an electromagnet 13 with a coil 20 and a core 18. It is constructed so as to fulfill the following formulas (1), (2), and (3).S1+S2.gtoreq.0.3.times.S3 (1)0.2.ltoreq.S2/(S1+S2).ltoreq.0.8 (2)1 mm.ltoreq.Dmax.ltoreq.5 mm (3)wherein S1 stands for the cross sectional area of the core 18, S2 for the cross sectional area of the coil 20, S3 for the cross sectional area of the sleeve 12, and Dmax for the maximum distance formed between the outer periphery of the holder 14 and the inner periphery of the sleeve 12.