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 (14-1) connected to a high frequency power generator, a second portion (14-2) connected to the first portion (14-1) and extended in the longitudinal direction of the workpiece , a third portion (14-3) connected to the second portion (14-2) and arranged surrounding a circumference of the general portion of the workpiece , a fourth portion (14-4) connected to the third portion (14-3) and extended in the longitudinal direction of the workpiece , and a fifth portion (14-5) connecting the fourth portion (14-4) and the high frequency power generator, and the fifth portion (14-5) is positioned on a more upstream side than the third portion (14-3) 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 (14-2) and the fourth portion (14-4) in a manner to cross the fourth portion (14-4). 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 (14-1) connected to a high frequency power generator, a second portion (14-2) connected to the first portion (14-1) and extended in the longitudinal direction of the workpiece , a third portion (14-3) connected to the second portion (14-2) and arranged surrounding a circumference of the general portion of the workpiece , a fourth portion (14-4) connected to the third portion (14-3) and extended in the longitudinal direction of the workpiece , and a fifth portion (14-5) connecting the fourth portion (14-4) and the high frequency power generator, and the fifth portion (14-5) is positioned on a more upstream side than the third portion (14-3) 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 (14-2) and the fourth portion (14-4) in a manner to cross the fourth portion (14-4). <EOS>