A lowermost Campanian nostoceratid ammonoid Eubostrychoceras valdelaxum was discovered, accompanying with Platyceramus japonicus, Sphenoceramus naumanni, and Damesites cf. damesi from the Kunitan Formation of the Kuji Group in the Iwate Prefecture, northeastern Japan. This is the first report of the occurrence of this species outside of the Yezo Group, Hokkaido, northern Japan. Eubostrychoceras valdelaxum occurs in a wide range of sedimentary facies in the multiple areas of the same horizon and might be beneficial for stratigraphic correlation of the lowermost Campanian (Platyceramus japonicus Zone) in the northwestern Pacific region. Besides E. valdelaxum, the ammonoids of the Kunitan Formation seem to have much in common with the species described from the Yezo Group, but more detailed understanding of the ammonoid fauna requires further accumulation of fossil records from the Kunitan Formation.