NOVELTY - Method for selective removal of chlorine and phosphorus from cellulosic biomass solid involves adding dispersed or semi-continuous liquid phase (A1) comprising base solution to vessel (A) of vessels each provided with detrimental species-containing cellulosic biomass solid, reacting, repeating the contacting process with vessel (B), adding dispersed or semi-continuous liquid phase (B1) to vessel (A), reacting, discharging basic water effluent, repeating the contacting process with vessel (B) and transferring portion of treated biomass to digestion and/or reactor.    USE - Method for selective removal of chlorine and phosphorus from cellulosic biomass solid e.g. wood and forestry residues, agricultural residue, herbaceous material, municipal solid waste, pulp and paper mill residue, corn stover, straw, bagasse, Miscanthus and Sorghum residue for producing ethanol, methanol, hydrogen, vegetable oil and methane.    ADVANTAGE - The method enables selective removal of chlorine and phosphorus from cellulosic biomass solid by economical process.    DETAILED DESCRIPTION - Method for selective removal of chlorine and phosphorus from cellulosic biomass solid involves adding dispersed or semi-continuous liquid phase (A1) comprising base solution having pH of more than 9 and flux of 1 kg/(m2.second) to vessel (A) of vessels each provided with detrimental species-containing cellulosic biomass solid, reacting at 0-60 degrees C, adding dispersed or semi-continuous liquid phase (A1) comprising base solution having pH of more than 9 and flux of 1 kg/(m2.second) to vessel (B) of vessels each provided with detrimental species-containing cellulosic biomass solid, reacting at 0-60 degrees C, adding dispersed or semi-continuous liquid phase (B1) comprising aqueous solution having pH of 8 or more to vessel (A), reacting, discharging basic water effluent, repeating the contacting of solution (B1) with vessel (B) and transferring portion of treated biomass to digestion and/or reactor. The total content of basic water effluent discharged from vessels is 0.5-3 parts mass with respect to 1 part mass detrimental species-containing cellulosic biomass solid.