To provide a continuous heating furnace capable of heating a billet from an ordinary temperature to a prescribed temperature, and shortening its temperature rising cycle even in the continuous heating furnace using induction heating coils to which non-resonant inverters are connected.

A billet heater 10 provided with the induction heating coils 20, 22 includes the induction heating coils 20, 22 disposed along a transporting passage 12 for transporting the billet 34, inverters 24, 26 independently connected to the induction heating coils 20, 22, and having load circuits of which coil inductance is changed accompanied by the transfer of properties of the billet 34, and a control section 28 for independently controlling the inverters 24, 26. The control section 28 detects electric currents and voltages charged to the induction heating coils 20, 22, calculates a frequency to have the set electric current when the plurality of inverters 24, 26 lead the electric current with rated voltages, and an operation is performed while keeping the frequency of the output electric current in each of the inverters 24, 26 at the calculated frequency.