Some Westerners might find this custom a bit different, but in Japan, it's considered a no-no to leave work before the boss. Whereas, in the West, "first one in and last one out" is considered to be a sign of a good work ethic in a boss, Japanese employees are expected to stay at work just as long as their bosses. Japan has a very work-centered culture, so it wouldn't be uncommon for employees to work extra hours if the boss isn't finished yet.
We suppose that you would just have to be hopeful that your boss isn't the type to stay late and work if you were an employee in a Japanese office.