A preparation is useful for treating renal disease, such as nephritis and especially nephrosis (nephrotic syndrome), which comprises an effective amount of N-acetylneuraminic acid shown by the formula ##STR1## wherein, when n is 1, Z represents hydrogens, lithium, potassium, sodium, ammonium or organic ammonium, and when n is 2, Z represents calcium, barium or magnesium, and a pharmacologically acceptable carrier.