To provide a method and a device for a high-frequency induction hardening capable of obtaining a hardened metallic material without forming an oxidized layer and having good surface characteristic.

In a hardening vessel 2, a gear 1 made of a steel member is laid on a loading table 20 and is positioned in an annular high-frequency induction coil 3. In the hardening vessel 2, an inlet 11 for nitrogen as an inert gas and an exhaust gas outlet 12 are provided, and the surface of the gear 1 is heated under the nitrogen atmosphere by the high-frequency induction coil 3. After heating to a prescribed temperature, the supply of electricity to the high-frequency induction coil 3 is stopped, and hardening liquid removed dissolved oxygen therein by blowing the gaseous nitrogen and bubbling at a removal treating part 7 for removing dissolved oxygen, is supplied into an annular hardening liquid supplying path 4 integrally arranged with the high-frequency induction coil 3 and injected to the heated gear 1 from its ejector 5 to execute the hardening.