To get a stable temperature distribution by restraining decline in temperature due to a gap between induction heating coils at the time of a line top.

A plurality of induction heating coils 3-1 to 3-n connected in parallel with an inverter power source 1 are arranged in series along the axis, inside which works (billets or the like) are guided in to be induction heated. Density of coil winding of both ends X, Z of the induction heating coils 3-1 to 3-n is made higher than that of a center Y. Magnetic field strength g' of gaps G between the induction coils is set at the same level as that y of the center part Y to compensate temperature decline of the works at parts corresponding the gaps G for leveling temperature distribution.