Sports betting is popular in Japan, and with a wide range of events available, like soccer, baseball, basketball, and even sumo wrestling, as a traveler you can easily place bets at one of the many licensed betting shops, or through online betting sites.
The bookmakers set their odds to make a profit whatever the outcome, the odds are based on money expected to be placed and statistical probability. Many OLBG member create their own betting odds before the bookies have priced up an event, it's a very worthwhile exercise and should become part of your betting life.