Although the national sport is Sumo, the most popular sport in Japan is baseball, which is not so common in Europe. Football, soccer is also popular but the difference between the popularity of football and baseball is that people are very keen on amateur baseball but not so much on amateur football.
The best professional baseball league is NPB, Nippon Professional Baseball which has two leagues, 6 teams each. The two leagues are the Central league and the Pacific league. Until 90s, the Pacific league had less popularity than the Central league which has two big teams, Yomiuri Giants and Hanshin Tigers. Giants has its franchise in Tokyo, home stadium is Tokyo dome and Tigers are in Nishinomiya city which is half way between Osaka and Kobe, and its home stadium is Hanshin Koshien stadium. These stadiums are usually packed with more than 40000 spectators per game.
Although the popularity was low, the Pacific league was said that the Pacific league is stronger than the Central leagues. Now all stadiums have more than 10000 spectators per game but it was may be 1/10 till 80s. Then the popularity of the Pacific league gradually became strong in 90s, especially when Hideo Nomo entered Kintetsu Buffaloes in 1989. Then, Ichiro Suzuki was drafted by Orix Bluewaves in 1991. In 1994, Ichiro Suzuki changed his registered name to Ichiro and starts his way to stardom. He finally hit then national record 210 hits in this year and became the youngest fielder to achieve the MVP of the year. Nomo and Suzuki both moved to the United States and became the superstars in MLB.
The popularity of Pacific leagues increased also thanks to good looking players and the teams taking care of the fans and the locals. Until 80s, professional baseball players looked like gangsters, wearing fancy clothes and jewelries and having short permed hairs.
The different thing between MLB and NPB is that teams of NPB are owned by Japanese big companies and have companies names as the team names. For example, Yomiuri Giants are owned by Yomiuri newspaper which is most daily published newspaper on Earth. Hanshin Tigers are owned by Hanshin Electric railways which is a private railway in Osaka/Kobe area. However, it is very trendy now to include city names to their team names. For example, Yakult Swallows changed its name to Tokyo Yakult Swallows.
During the season, between April and October, you will meet many baseball fans on streets, on the trains and of course at the pubs.