In Japan, there remain several regions where the air quality standard for PM_2.5 have not been attained, and the Seto Inland Sea is one of them. To consider the reason for the high PM_2.5 concentration in this region, the vertical profile of meteorological parameters and PM_2.5 concentration were measured by drone at a site facing the Seto Inland Sea. We collected a total of 15 times measurements at 10, 30, 100, 300, 500 m a.s.l. during the 2020 autumn. The highest PM_2.5 concentration was measured by drone at 6 AM on October 14. This high concentration was also observed by the nearest Atmospheric Environmental Regional Observation System (AEROS) measurement sites. Over the Seto Inland Sea, a higher PM_2.5 concentration was observed from evening to night over Okayama prefecture, and subsequently observed during the morning over Kagawa prefecture. Combining the result of numerical modeling simulation, the stagnant air between the Chugoku and Shikoku mountains is a possible factor to cause high PM_2.5. In addition, the long-range transport from the Asian continent is also an important factor to enhance PM_2.5 in this region.