PURPOSE: An aqueous unsaturated amide is neutralized with a caustic alkali to effect reaction and subjected to reverse osmotic treatment to produce the titled aqueous solution used as a raw material for anionic high-molecular-weight flocculating agent readily and effectively without any trouble of polymerization.

CONSTITUTION: An aqueous unsaturated amide containing unsaturated organic acid is neutralized with a caustic alkali to convert unsaturated organic acid into the metal salt, which is separated with a reverse osmotic membrane to give the titled aqueous solution. The unsaturated amide means an amide containing a polymerizable unsaturated bond such as acrylamide. The unsaturated organic acid means the one corresponding to the acid formed by the hydrolysis of the unsaturated amide such as acrylic acid.

EFFECT: When the unsaturated amide is neutralized with a caustic alkali, the degree of membrane's exclusion of the unsaturated organic acid is remarkbaly raised to enable the removal.