Plant megafossils from the early Middle Miocene marine sediments in Tatsugoroshi of Hitachiota, north-central part of Ibaraki Prefecture are described. The plant-bearing siltstone and sandstone beds are in the Tatsukuroiso Mudstone Member of the Higashikanasayama Formation, and is assigned to the lower part of the Denticulopsis lauta zone in the diatom biostratigraphy. Total 17 types of plant megafossils are briefly described, and floral characters of the Tatsukuroiso assemblage are compared with the Daijima-type floras of late Early and earliest Middle Miocene. The assemblage has some common elements to the Daijima-type floras, but unusually includes many leaves of Fagus antipofii. Co-occurred marine molluscs, echinoid and siliceous sponge indicate the Tatsukuroiso Mudstone Member has deposited in relatively deep-sea environments. The Tatsugoroshi plant megafossil assemblalge represents one of the plant taphocoenosis examples preserved in marine sediments.