A reactor has a casing composed of a magnesia-based refractory, and a bottom plate composed of an MgO-graphite mixed refractory is disposed on a bottom part of this casing. A graphite crucible is provided at a bottom of the reactor, and the graphite crucible and the reactor are joined together by a magnesia cylinder. Iron ore powder, coal powder, and other such raw materials supplied into the reactor are irradiated with microwaves from microwave oscillators and are heated. The iron ore is reduced, and the resulting molten pig iron flows out through a hole into a crucible, and then is poured out of the crucible through another hole into a ladle. It is thereby possible to manufacture molten pig iron with high energy efficiency, instead of using blast-furnace iron-making.