Oxadiazole derivatives represented by formula (I): (wherein R1 represents a hydrogen atom or an amino-protective group; R2, R3, and R4 each independently represents an alkyl group, a cycloalkyl group, a phenyl group which may be substituted, or a 3,4-methylenedioxyphenyl group, or R3 and R4 are taken together to represent a C2-6 alkylene group), a process of producing the same, and a process for producing oxadiazole derivatives represented by formula (II): (wherein all symbols have the same meanings as described above) using the above derivative. According to the invention, the compound represented by formula (II) is produced through fewer steps in a high yield.