To provide a melting furnace in which a base substrate is immersed in a melt to stably supply an additional raw material without stopping production of a crystal thin plate to be formed on the surface of the base substrate, and degradation and fluctuation in the quality are decreased, as well as the production yield of the crystal thin plate is improved, and thereby, productivity of the crystal thin plate can be improved.

The melting furnace includes: a main crucible for holding a melt; a main crucible heating device for heating the melt in the main crucible; a sub-crucible for holding a melt to be supplied the main crucible; a sub-crucible heating device for heating the melt in the sub-crucible; a solid raw material injection device for injecting a solid raw material; a melt transfer part for supplying the melt from the sub-crucible to the main crucible; and a solid raw material charging control apparatus which determines whether the solid raw material has been injected into the sub-crucible from the solid raw material injection device.