If you want to watch a sport game in Japan, definitely baseball game is the one which you should watch. Baseball is the most popular professional sport in Japan and NPB is the top professional league.
Actually NPB consists of two leagues. Central league and Pacific league, which has 6 teams in each league. Then, these teams are spread nationwide, from Sapporo to Fukuoka. Tokyo metropolitan area has five teams, three in the Central league and two in the Pacific.
The one of the two teams of Pacific league is Saitama Seibu Lions, which stadium is located in Tokorozawa - the city which I grew up. It is approximately 30 km north west of Tokyo, 30 minutes train ride from Seibu Ikebukuro station.
The stadium was used to be open air stadium when it opened in 1979 as Seibu stadium but roof was placed in 1999 and renamed to Seibu dome. Therefore the games will not be cancelled except in case of extremely bad weather - typhoon is coming or large earthquake happened. Uncovered stadium can be cancelled easily due to normal rain. Seibu dome is now called Metlife dome due to Metlife insurance pays the naming rights.
For tourists coming to Japan, it is sad when your game was called off - cancelled due to a normal rain. Therefore it is better to watch a game at the covered stadium - dome stadium. There are 6 dome stadium in Japan, Sapporo, Tokyo, Nagoya, Osaka, Fukuoka and Metlife domes.
Tokyo dome is a home for Yomiuri Giants of Central league which is the most popular team in Japan which attracts at least 40 thousand fans per game. The tickets can be sold out quickly, therefore it is better to watch the Seibu Lions' game at Metlife dome.