A fixing device comprising: a heating member including an induction heat generation layer; a pressurizing member to be pressed to the heating member at a predetermined pressure; and an induction heating portion configured to heat the induction heat generation layer of the heating member by induction heating by magnetic flux, the fixing device configured to pass a recording medium through a nip portion between the heating member and the pressurizing member, to perform fixing processing for a toner image, and the induction heating portion including: a coil that is wound in a loop manner along a longitudinal direction of the heating member and generates magnetic flux for heating the heating member by induction heating; a magnetic core that is arranged, in the vicinity of the coil, in a direction perpendicular to a conveyance direction of the recording medium, and conducts the magnetic flux into the induction heat generation layer of the heating member; a coil bobbin provided opposing to a surface of the heating member, the coil being attached on an attachment surface of the coil bobbin opposite to a surface thereof opposing to the heating member; a core holder provided opposing to the coil bobbin, the magnetic core being attached on the core holder; a cover member attached to the coil bobbin so as to cover the magnetic core and the coil; and an auxiliary duct provided between the cover member and the core holder, the auxiliary duct communicating with an intake hole provided at one end in a longitudinal direction of the cover member, and the auxiliary duct having opening holes overlapping with some of a plurality of holder opening portions formed on the core holder, wherein air is caused to flow into between the core holder and the coil bobbin via the auxiliary duct and the holder opening portions from the intake hole, and the air warmed by heat of the coil is discharged from an exhaust hole formed at the other end of the cover member, thereby adjusting the flowing amount of air flowing in a longitudinal direction of the coil. a heating member including an induction heat generation layer; a pressurizing member to be pressed to the heating member at a predetermined pressure; and an induction heating portion configured to heat the induction heat generation layer of the heating member by induction heating by magnetic flux, the fixing device configured to pass a recording medium through a nip portion between the heating member and the pressurizing member, to perform fixing processing for a toner image, and the induction heating portion including: a coil that is wound in a loop manner along a longitudinal direction of the heating member and generates magnetic flux for heating the heating member by induction heating; a magnetic core that is arranged, in the vicinity of the coil, in a direction perpendicular to a conveyance direction of the recording medium, and conducts the magnetic flux into the induction heat generation layer of the heating member; a coil bobbin provided opposing to a surface of the heating member, the coil being attached on an attachment surface of the coil bobbin opposite to a surface thereof opposing to the heating member; a core holder provided opposing to the coil bobbin, the magnetic core being attached on the core holder; a cover member attached to the coil bobbin so as to cover the magnetic core and the coil; and an auxiliary duct provided between the cover member and the core holder, the auxiliary duct communicating with an intake hole provided at one end in a longitudinal direction of the cover member, and the auxiliary duct having opening holes overlapping with some of a plurality of holder opening portions formed on the core holder, wherein a coil that is wound in a loop manner along a longitudinal direction of the heating member and generates magnetic flux for heating the heating member by induction heating; a magnetic core that is arranged, in the vicinity of the coil, in a direction perpendicular to a conveyance direction of the recording medium, and conducts the magnetic flux into the induction heat generation layer of the heating member; a coil bobbin provided opposing to a surface of the heating member, the coil being attached on an attachment surface of the coil bobbin opposite to a surface thereof opposing to the heating member; a core holder provided opposing to the coil bobbin, the magnetic core being attached on the core holder; a cover member attached to the coil bobbin so as to cover the magnetic core and the coil; and an auxiliary duct provided between the cover member and the core holder, the auxiliary duct communicating with an intake hole provided at one end in a longitudinal direction of the cover member, and the auxiliary duct having opening holes overlapping with some of a plurality of holder opening portions formed on the core holder, wherein air is caused to flow into between the core holder and the coil bobbin via the auxiliary duct and the holder opening portions from the intake hole, and the air warmed by heat of the coil is discharged from an exhaust hole formed at the other end of the cover member, thereby adjusting the flowing amount of air flowing in a longitudinal direction of the coil.