We would like to investigate the strong wind phenomenon effecting to the onset of a sea breeze. It is general the fact that the onset time of a sea breeze is mainly affected according to the distance from the coastline but we find the reversal fact. The onset time at Suyoung is faster than that at Haeundae in spite of the observation site of Suyoung is 5 m and that of Haeundae is 1 km away from the coastline. This is the reason that the nighttime air is converged the lower area by surface cooling and then it is strongly drained onto the lowest area, Suyoung river until the sunrise. it is proved by observation data at Suyoung and Haeundae.