A fire proof treatment solution capable of resolving problems, such as low solubility leading to low concentration and hence low combustion inhibiting effect and water solubility leading to a disadvantage of eluviation, of boron compounds used as a constituent of flame retarding treatment solution. In particular, a fire proof treatment solution produced by adding phosphoric acid and a silane coupling agent to a weak acid or weak alkali aqueous metal salt solution containing a boron compound to thereby obtain a mixed aqueous solution and adding a plant extract composed mainly of a polyphenol compound to the mixed aqueous solution. This fire proof treatment solution can be widely applied to members composed mainly of cellulose, such as wood, paper and natural fibers. This fire proof treatment solution can increase the fixation of chemicals to thereby attain a high combustion inhibiting effect and can minimize the occurrence of eluviation.