To produce a denitrating reductant composition consisting of a high-purity urea aqueous solution by efficiently removing ionic impurities from the high-concentration urea aqueous stock solution prepared by using an inexpensive raw material easily available commercially.

The denitrating reductant composition consisting of the high-purity urea aqueous solution having 30-50 mass% urea concentration, pH≤8.0, ≤500 μS/cm electric conductivity and ≤0.5 ppm concentration of each of Ca, Fe, Na and K is produced by making the urea aqueous stock solution of 30-50 mass% urea concentration pass continuously through a single bed column of an H-type strongly acidic cation exchange resin.