We describe a new deep-sea arcid species, Bentharca steffeni sp. nov., from the Paleocene Katsuhira Formation in Urahoro Town, eastern Hokkaido. This is the oldest certain record of this genus. Shell microstructure of this new species is similar to the Recent species, B. asperula (Dall, 1891) in having an outer layer composed of thin simple lamellar fibrous prismatic and crossed lamellar structures and an inner layer composed of irregular complex crossed lamellar and irregular prismatic structures. Of these, the simple lamellar fibrous prismatic and the irregular prismatic structures have never previously been recognized even in B. asperula. From the occurrence of the new species, it has been elucidated that the genus Bentharca adapted to the deep sea as a refuge at least in the Paleocene.