To control the falling quantity of metal material from a preheating section to a melting section constantly within an optimal range by disposing the preheating section above the melting section provided with an oxygen burner, disposing a restrictor between the melting section and the preheating section and disposing a water cooled pin at the lower part of the restricting section or the preheating section.

A melting section 2 is disposed at the lower part of a metal melting furnace and a preheating section 3, comprising a tubular body having smaller diameter than the melting section 2, is disposed at the upper part of the melting section 2 while being coupled integrally therewith a restricting section 6 is disposed between the melting section 2 and the preheating section 3 and a water cooled pin 4 is provided movably at the lower part of the restricting section 6 or the preheating section 3 such that the length being inserted into the furnace from the out side can be regulated. A cover body 5 having an exhaust port 5a is fixed removably to the upper opening 3a at the preheating section 3. The or tour water cooled pins 4, for example, are set movably along the radial direction of the furnace through a driver such that the forward ends thereof can be arranged at appropriate positions in the preheating section 3.