The tandem action of serine protease (α-chymotrypsin or subtilisin BPN') and Aspergillus genus aminoacylase on racemic N-acetyl-α-aminoalkanedioic acid α, ω-diester produced L-α-aminoalkanedioic acid ω-ester in good yield and high optical purity. L-α-Aminosuberic acid ω-ester thus obtained was conveniently introduced into an oxytocin analog, [Asu^{1, 6}]oxytocin, by the solid-phase-synthesis and cyclization-cleavage method with oxime resin.