Two men meet in a ring that is 4.55 metres across. The idea of the game is to force your opponent out of the ring or make a part of his body - apart from the soles of the feet - touch the floor.
Fighting can last just a few seconds or as long as three minutes.
Pushing, tripping, slapping and body throws are in. But pulling hair, gouging eyes, and punching are all out.
The referee, with the help of judges, watches the match and decides who the winner is.
In sumo you can only increase your rank (and the amount of money you earn) by winning. If you lose, you drop in rank.
Wrestlers wear special clothing - in the ring they wear thick silk belts around their waists, and have their hair carefully styled according to their rank.