A high frequency induction heating apparatus comprising: a high frequency induction heating coil for heating a long hollow steel workpiece and forming a high temperature portion in the workpiece which moves in a longitudinal direction of the workpiece, the workpiece having a closed cross section and an outward flange, the outward flange extending in the longitudinal direction of the workpiece and defining two faces (12a-1, 12a-2), wherein the high frequency induction heating coil comprises a magnetic material core connected to an induction heating coil, wherein the induction heating coil comprises a first portion connected to a high frequency power generator, a second portion connected to the first portion and extended in the longitudinal direction of the workpiece, a third portion connected to the second portion and arranged surrounding a circumference of the general portion of the workpiece, a fourth portion connected to the third portion and extended in the longitudinal direction of the workpiece, and a fifth portion connecting the fourth portion and the high frequency power generator, and the fifth portion is positioned on a more upstream side than the third portion in the opposite direction from a moving direction of the high temperature portion of the workpiece; wherein the magnetic material core comprises two portions (13-1, 13-2) parallel to the outward flange provided on a downstream side in the moving direction of the high temperature portion of the workpiece, the two portions (13-1, 13-2) being for generating a magnetic flux penetrating the two faces of the outward flange, and the magnetic core is arranged between the second portion and the fourth portion in a manner to cross the fourth portion. a high frequency induction heating coil for heating a long hollow steel workpiece and forming a high temperature portion in the workpiece which moves in a longitudinal direction of the workpiece, the workpiece having a closed cross section and an outward flange, the outward flange extending in the longitudinal direction of the workpiece and defining two faces (12a-1, 12a-2), wherein the high frequency induction heating coil comprises a magnetic material core connected to an induction heating coil, wherein the induction heating coil comprises a first portion connected to a high frequency power generator, a second portion connected to the first portion and extended in the longitudinal direction of the workpiece, a third portion connected to the second portion and arranged surrounding a circumference of the general portion of the workpiece, a fourth portion connected to the third portion and extended in the longitudinal direction of the workpiece, and a fifth portion connecting the fourth portion and the high frequency power generator, and the fifth portion is positioned on a more upstream side than the third portion in the opposite direction from a moving direction of the high temperature portion of the workpiece; wherein the magnetic material core comprises two portions (13-1, 13-2) parallel to the outward flange provided on a downstream side in the moving direction of the high temperature portion of the workpiece, the two portions (13-1, 13-2) being for generating a magnetic flux penetrating the two faces of the outward flange, and the magnetic core is arranged between the second portion and the fourth portion in a manner to cross the fourth portion.