Thank you for your reply, I just zoom into the sequence, click to highlight and move the clips one by one, also adjust their begins and ends to nearest 1/100 second (eg. 10.020 - 12.030 rather than 10.024 - 12.038 etc.) I saw this in one of the post, but black spaces still occur.
At the beginning the black spaces are visible in sequence preview and exported videos, later I tried to round up the boundaries like above, sequence preview showed no problem but when I export it, spaces were still here:(