We investigated the deep-sea sedimentary sequence of core MD012422 corresponding to the last 350 ka (since marine isotopic stage, MIS 10) from off Shikoku Island, NW Pacific Ocean, and refined its tephrostratigraphy. We detected many tephras and cryptotephras and correlated them with terrestrial tephras on the basis of the major- and trace-element compositions of their glass shards. Then we cross-checked our results against the reported marine tephrostratigraphy in the NW Pacific area and assessed the relative timing of tephra eruptions and bioevents. Many widespread tephras and cryptotephras originated from the Kyushu volcanic zone (Kuju, Aso, Kakuto, Aira, Ata, and Kikai volcanoes/calderas), along with the Takayama-Ng1 tephra (Tky-Ng1) from Suiendani volcano in central Honshu, were detected in the core. These tephras were used to validate and refine the reported NW Pacific marine tephrostratigraphy and to develop useful time-synchronous markers for this ocean area. Among the tephras, Ata-Torihama (Ata-Th) occurs at the MIS 7/8 transition (ca. 240 ka) and its horizon overlaps with a calcareous nannofossil biohorizon, the first appearance datum (FAD) of Emiliania huxleyi. Ata-Th and this biohorizon are stratigraphically above the Tky-Ng1 (MIS 8/9 transition, 290–300 ka) and Kakuto (Kkt, MIS 9/10 transition, 330–340 ka) tephras, and these tephras have the same stratigraphic relationship with this biohorizon in deep-sea sediments from the East China Sea, off Shikoku Island, and off central Honshu. However, in deep-sea cores from off northern Honshu, an inconsistency between the tephrostratigraphy and the FAD of E. huxleyi has been reported. This inconsistency can be explained by poor preservation of calcareous fossils in the deep waters off northern Honshu, which are more corrosive to calcium carbonate than the shallower waters off Shikoku Island. These findings emphasize the importance of cross-checking biohorizons against tephra horizons and of linking tephras between terrestrial and marine sequences to assess the synchronicity of globally correlated age indicators.