Sialic acid derivatives represented by the general formula (I): ##STR1## wherein R.sup.1 is a steroidal compound residue; R.sup.2 is H or alkyl; R.sup.3 is alkyl; ##STR2## wherein each of R.sup.6 and R.sup.7 is H, alkyl or the like and I is an integer of 0 to 6; or the like; X is O or S; R.sup.4 is H or acyl; and R.sup.5 is R.sup.14 O-- (R.sup.14 is H or acyl) or R.sup.15 NH--(R.sup.15 is acyl or the like); their salts, hydrates or solvates are provided. Sialic acid derivatives of the present invention are expected to be effective medicines for the prevention and therapy of senile dementia including Alzheimer's disease and the like, because they increase ChAT activity in cholinergic neurons.