We have been continuously observing the daily Be-7 concentrations at Bangkok in Thailand ($13.765^\circ$ N, $100.526^\circ$ E) since June 2014 to investigate the relationship between solar activities and the variation of cosmogenic nuclides in low latitude. The Be-7 concentrations show a clear seasonal variation with the high and low concentrations during January to May and June to December, respectively, for each year. In order to understand the seasonal variations, we calculated back trajectories of air mass by HYSPLIT. The calculation indicates that the back trajectories are distinguished three patterns in a year, which are "Indian Ocean trajectories", "Continental trajectories", and "Pacific Ocean trajectories". Moreover, we estimated monthly amount of Be-7 coming to Bangkok for each trajectories by EXPACS. From these findings, the amount of Be-7 is high during Continental and Pacific Ocean trajectories. On the other hand, one is low during Indian Ocean trajectories. This phase of calculated Be-7 can explain the seasonal variation. Comparing this phase and the phase of observed Be-7 concentrations, there is a time lag of one or two months.