In Japan you are supposed to enter the bus from the rear door and exit from the front door. You pay as you exit rather than when you board. If you have an all-day pass, the first time you use it you put it into the specified slot and it will print a date on it. After that all you have to do is show the date to the driver whenever you exit the bus. Note that the seats toward the front are generally reserved for the elderly, injured, and pregnant. They will be marked by a sign with pictures for those who can't read Japanese.