An oxadiazole derivative of formula (I) and a non-toxic salt thereof, ##STR00001## wherein R is hydrogen, alkyl, CycA, etc.; AA.sup.1 is a single bond, amino acid residue, etc.; AA.sup.2 is a single bond, amino acid residue, etc.; R.sup.7 and R.sup.8 are hydrogen, alkyl, etc.; R.sup.9 is hydrogen, alkyl, etc. R.sup.10 is hydrogen, alkyl, etc.).