A widespread bloom of the harmful raphidophyte Chattonella ovata Y. Hara & Chihara occurred for the first time in the east Seto Inland Sea in the summer of 2016. From July 11 to 17, cell densities in Hiuchi-Nada and Bisan-Seto reached 10 cells mL^-1. From July 19 to 24, the highly dense distribution area expanded to Harima-Nada, and cell densities reached their peak (max. 222 cells mL^-1) in all areas affected from July 25 to 31. In Harima-Nada, the species decreased rapidly, whereas the level at Hiuchi-Nada and Bisan-Seto was maintained until August. From July 11 to 18, a westerly wind had prevailed in the Takamatsu area facing Bisan-Seto while the wind was weak during other periods. That caused eastward wind-induced passage flow, and expansion of the cell distribution from Bisan-Seto to Harima-Nada. In addition, the sunlight hours were long (average 8.6 h) and the water temperature was high (approximately 25 to 29˚C)．Furthermore, salinity remained at around optimum for C. ovata (approximately 28 to 31) in the east Seto Inland Sea while the species bloomed. This indicates that these environmental conditions were suitable for growth of this species, which is adapted to strong irradiance, high water temperature and high salinity. In Bisan-Seto and Hiuchi-Nada, Chattonella marina and Chattonella antiqua appeared at high cell densities in mid-July when the distribution of C. ovata expanded. However, in Harima-Nada, C. marina and C. antiqua maintained low cell densities consistently. In Harima-Nada, DIN （dissolved inorganic nitrogen） and DIP (dissolved inorganic phosphorus) remained at 0.51–1.17 µM and 0.08–0.25 µM, respectively, from July 4 to August 15. The minimum cell quotas of nitrogen for C. ovata are lower than for C. antiqua. We presumed this gave a competitive advantage to C. ovata for growth. In Harima-Nada, cell density of diatoms decreased temporarily in late-July, but the density remained above 100 cells mL^-1 after August. However, in Hiuchi-Nada and Bisan-Seto, cell densities of diatoms were kept below 100 cells mL^-1 through the observation period. We inferred that a low concentration of nutrients, less westerly wind, and a high density of diatoms were factors that led to the regulation of the low level of C. ovata after August in Harima-Nada. We considered that the low diatom density was one of the causes of the C. ovata bloom being prolonged in Hiuchi-Nada and Bisan-Seto.