The cucumariid sea cucumber Plesiocolochirus inornatus (Marenzeller, 1881) was rediscovered in shallow water of the Pacific Ocean, on the south coast of Fukushima Prefecture, eastern Japan. The last report of this species in Japanese waters was published a hundred years ago, although this species possibly has a wide distribution in Japanese waters. Since there is no detailed information on its external appearance, we prepared images of the body shape on the basis of six new specimens. The body of P. inornatus has obviously distinguishable dorsal and ventral sides in the living state with tapered ends that turn upwards, mid-body of the ventral side extremely flattened and nearly half-round in cross-section, dorsal surface smooth with retracted minute ambulacral-papillae scattered uniformly regardless of radii or inter-radii. Furthermore, detailed images of ossicle shape and calcareous ring shape were prepared.