To melt a metal material efficiently only through an oxygen burner by specifying the combustion amount of the oxygen burner during an interval after the metal material is thrown into a preheating section before the metal material is dropped entirely into a melting section thereby optimizing the quantity of heat being thrown into the melting section.

When a metal material is thrown into a preheating section 3 through an upper opening thereof, a part of the metal material is passed through a reduced section 4 and dropped into a melting section 2 and the remainder is left in the preheating section 3. The metal material dropped into the melting section 2 is then melted by firing an oxygen burner 1 and high temperature exhaust gas produced through melting ascends through the reduced section 4 to the preheating section 3 and preheats the metal material thereat. The combustion amount of the oxygen heater per 1 ton of the metal material is controlled to 100-450 Mcal/h.t per hour during an interval after the metal material is thrown into a preheating section 3 before the metal material is dropped entirely into the melting section 2.