PURPOSE: To perform the reverse osmosis treatment of raw water containing silicic acid generating no contamination of a membrane due to silicic acid, by using a composite semi-permeable membrane having an ultra-thin membrane layer formed by crosslinking a polyfunctional amino compound having two or more of secondary amino groups in one molecule thereof by a crosslinking agent.

CONSTITUTION: A polyfunctional amino compound having two or more of secondary amino groups in one molecule thereof or a solution prepared by mixing polyvinyl alcohol with said amino compound is applied to a porous base material having minute pores with a pore size of 10W100 formed to the surface thereof to impregnate said base material. the concn. of said amino compound is 0.05W10wt% and, when polyvinyl alcohol is used, said polyvinyl alcohol is used in an amount of 0.2W5 times of the amino compound. A polyfunctional crosslinking agent is contacted with the treated base material and the whole is heated at 80W180°C to form an ultra-thin membrane layer with a thickness of 50W800. The concn. of the crosslinking agent is pref. 0.05W10wt%.