This invention relates to a method for removing fumarase activity from a microorganism or processed product thereof having ethylenediamine-N,N'-disuccinic acid ethylenediamine lyase activity, which includes treating the microorganism or processed product thereof with an aqueous alkaline solution at a pH of 8.0 to 10.5 in the presence of at least one salt with a concentration of 5 mM to 1000 mM. The salt is preferably selected from the group consisting of sodium, potassium, ammonium and C.sub.2-6 alkanediamine salts of boric acid, phosphoric acid, hydrochloric acid, sulfuric acid, acetic acid, oxalic acid, fumaric acid, maleic acid and ethylenediamine-N,N'-disuccinic acid, and mixtures thereof. This invention also relates to a microorganism or processed product thereof having reduced fumarase activity obtainable by the above described method, and to a method for producing an optically active aminopolycarboxylic acid from fumaric acid and a compound having amino group in the presence of the microorganim or processed product thereof having reduced fumarase activity as a catalyst.