PURPOSE: To obtain cephacetrile at room temperature quickly in high yield useful for remedying diseases caused by Staphylococcus aureus, etc., by reacting a silyl derivative of 7-aminocephalosporanic acid with a cyanoacetic ester derivative, followed by removing the silyl group from the reaction product.

CONSTITUTION: (A) 1mol silyl derivative of 7-aminocephalosporanic acid shown by the formulaI(R is trialkylsilyl) obtained by reacting 7-aminocephalosporanic acid with a silylating agent such as N,O-bistrimethylsilylacetamide, etc. is reacted with (B) 1W3mol cyanoacetic ester derivative shown by the formula CN- CH2-CO-Y (Y is halogen-substituted phenoxy having 4 or less substitution number, halogen-substituted phenylthio having 4 or less substitution number, 2- benzothiazolethio, etc.), and the reaction product is reacted with water or an alcohol and the silyl group is removed, to give the desired compound shown by the formula II.