In the history of baleen whales, the early Miocene is one of not a well-known time. Many baleen whales from the Miocene are known as “cetotheres”. The “cetothere” Isanacetus laticephalus was a baleen whale from the early to middle Miocene that is only known from Japan. The holotype and paratype of I. laticephalus are well-preserved, and the age of the species I. laticephalus makes it one of the earlier basal plicogulans; however, its phylogenetic position is unclear. Here, a new cranium including the periotic from an early Miocene deposit (about 18.5 to 17.0 Ma) of the Oi Formation, Ichishi Group in Mie, Japan is reported as cf. I. laticephalus. The specimen displays a combination of four periotic features: (i) a large lateral tuberosity; (ii) a rectangular pars cochlearis; (iii) a robust neck of the posterior process, and (iv) a cerebral aperture of the facial canal that is smaller than that of the dorsal vestibular area. This combination of features could be diagnostic for I. laticephalus, among basal plicogulans from the early and middle Miocene. Adding more referred specimens will expand the diagnoses of I. laticephalus, and firmly support phylogenetic trees with this species.