A fixing device includes a fixing member, which includes heat-generating layer, and an induction heater to inductively heat the fixing member. The induction heater includes an excitation coil disposed facing an outer circumferential surface of the fixing member to generate a magnetic flux, a ferromagnetic core assembly containing a ferromagnetic core to form a magnetic path to direct the magnetic flux generated by the excitation coil to the fixing member, and a holder to hold the excitation coil and the ferromagnetic core assembly. The ferromagnetic core is insert-molded in and covered by the holder. The holder has a plurality of spherical marks created by a plurality of stabilizing members each having a spherical tip to stabilize the ferromagnetic core in a mold.