To obtain a protein complex, modified in functional activities under weak acidic conditions and having high safety by bonding a protein or its hydrolyzate to galacturonic acid or polygalacturonic acid according to aminocarbonylating reaction.

This protein complex is obtained by bonding (A) a protein or its hydrolyzate to (B) galacturonic acid or polygalacturonic acid according to aminocarbonylating reaction. An albumen protein that is a heat coagulable protein or casein, etc., which is an acid precipitating protein is preferred as the ingredient (A). A substance prepared by hydrolyzing pectin or pectic acid with a pectinase and regulating the average polymerization degree to 2-15 is preferred as the ingredient (B). The objective complex is preferably concretely prepared by drying a mixed solution of the ingredients (A) with (B), adjusting the moisture content to ≤15% and reacting the resultant powder at 40-95°C within 4 weeks. The aminocarbonylating reaction is preferably carried out by regarding 15-85% residual amount of amino groups based on that before the reaction the end point. The ingredient (B) in an amount so as to provide ≥0.2 the number of reducing terminals based on that of amino groups in the ingredient (A) is preferably used.