To provide a method for removing excess amounts of water-soluble amine efficiently from a Mannich condensation product to improve formulating characteristic of the Mannich condensation product.

This method for removing excess amounts of water-soluble amine from a diluted crude Mannich condensation product comprising about 40 to 80 wt.% crude Mannich condensation product in an inert solvent comprises the steps of a) filtering the diluted crude Mannich condensation product in the existence of magnesium silicate and water, and further in the existence of a filter aid when an average particle size of the magnesium silicate in the particle size distribution comes less than about 50 μm, and b) recollecting the filtrate containing the Mannich condensation product having less than about 0.05 mEg/g water-soluble amine.