The method of preparing rice hull active carbon comprise recovering a useful metal element, i.e. silicon from rice hull generated as by-product in hulling, and directly reacting the silicon-eluted rice hull with NaOH in active furnace. Rice hull is compressed and crushed, and then is carbonized with an exterior or interior heat system at 500-700 deg. C. The carbonized rice hull is boiled with aquous alkali solution to recover SiO2 and then the SiO2-eluted carbonized rice hull is activated with aqueous NaOH solution. If the carbonized rice hull is mingled with the above pH 13 of aqueous alkali solution and the mixture is boiled, a solid flux of sodium amorphous silicate is formed while being eluted the SiO2 component of carbonized rice hull, and the silicon of a metal raw material can easily recovered from carbonized rice hulls. Said solid flux is a raw material of various synthetic chemicals using the silica gel or mordenite and silicon. Also, a process of activating the SiO2-eluted carbonized rice hull is pe
