A radioactive waste (zeolite to which Cs-137 was adsorbed) in a waste tank and a glass raw material (soda lime glass) in a glass raw material tank are supplied into a solidifying vessel. Graphite in a graphite tank is also supplied into the solidifying vessel. The solidifying vessel is filled with a mixture of the radioactive waste, glass raw material, and graphite and is then disposed in an adiabatic vessel. The radioactive waste and glass raw material in the adiabatic vessel are heated by thermal energy generated due to radiation emitted from Cs-137. The heat is transferred to the peripheral portion of the solidifying vessel through the graphite, raising the temperature of the peripheral portion. The glass raw material is melted and enters clearances among the radioactive waste, producing a vitrified radioactive waste. This radioactive waste solidification method can shorten a time taken to produce a vitrified radioactive waste.