The so-called broad tapeworm, which had been treated as Diphyllobothrium latum in Japan since 1889,was revised taxonomically and described as a new species, Diphyllobothrium nihonkaiense sp. nov., being distinguished from the original Diphyllobothrium latum in Finland. The taxonomic status of the new species among related species of the genus Diphyllobothrium is briefly discussed.