To obtain a sialic acid derivative for remedying disturbance of memory involved in senile dementia, by reaction of a specific carboxyl-bearing sialic acid derivative with an amine derivative in the presence of a phosphorus- based condensation agent and an organic base.

This sialic acid derivative of formula III is obtained easily at low cost in high yield and purity by amidation between a carboxyl-bearing sialic acid derivative of formula I [R2 is H or an acyl; R3 is a 1-15C alkyl or (substituted) aryl group-substituted 1-8C alkyl; R5 is acetamido group or alkyl group-substituted amino group; X is O or S] and an amine derivative of formula II (R1 is a steroid compound residue; R4 is H or a 1-4C alkyl) (e.g. aminocholestane) in the presence of a phosphorus-based condensation agent (e.g. phosphorus oxychloride) and an organic base (e.g. triethylamine). This sialic acid derivative is useful as e.g. a medicine for remedying disturbance of memory involved in senile dementia including Alzheimer's disease.