In storing of radioactive wastes by drying, pulverization and pelletizing, radioactive waste pellets are solidified with an alkali silicate solution as a filler, a substance having an action to harden the alkali silicate solution, and a substance having an action to absorb the water formed by the hardening reaction of the alkali silicate solution, or a substance having both actions to harden the alkali silicate solution and to absorb the water formed by the hardening reaction. The solidification can be attained with easy operation and prolonged stability at a low cost.