Isocubanite was found in spinel lherzolite xenolith with clinopyroxene-spinel symplectite from Ichinome-gata, Akita Prefecture, Japan. It occurs as two types; one is as exsolution lamella type in monosulfide solid solution (mss) and the other is a rim type observed around mss. The former type's chemical composition is very close to the ideal formula of isocubanite, CuFe2S3. The latter type has an intermediate composition between the ideal formula of chalcopyrite and that of isocubanite. The mss blebs including isocubanite lamellae associate commonly with orthopyroxene and clinopyroxene, sometimes with chromian spinel, and rarely with pargasite and clinopyroxene-spinel symplectite. The mss associating the rim type isocubanite occurs in orthopyroxene, clinopyroxene or olivine. The Ni-contents of mss associated with isocubanite are in the range from 19.3 to 25.8 of Ni/(Ni+Fe+Cu) in atomic%, and show the Cu contents vary from 0.07 to 0.45. Textures and chemical compositions of mss and isocubanite suggest that isocubanite occurred as an exsolution product from the originally formed monophase of mss, which had been crystallized during the metasomatic process in the upper mantle.