Sumo wrestling is a sport taken very seriously in Japan. One only has to look at the uniform of the gyoji (the referee) to get an idea of how seriously the outcome of a match is viewed. The uniform also indicates the ranking of the gyoji.
The gyoji visibly carries a traditionally made Japanese sword with a short blade (called a tantō) in his belt. These swords were also worn by the samurai class of feudal Japan. The blade is single- or double-edged, with a length between 6 and 12 inches.
The reason the gyoji carries this sword is to show he understands the seriousness of the decisions he has to make to fairly judge who the winner of a bout will be.
It also shows his willingness to commit seppuku if he makes a mistake! When translated, seppukumeans "stomach cutting," and it is a form of Japanese ritual suicide by disembowelment. The ritual suicide, meant to be performed in front of spectators, consists of plunging the tantō into the abdomen and slicing from left to right. Talk about a blood sport!
Thankfully, these days if a gyoji's decision about the match outcome is overturned by the shimpan, he is just expected to turn in his resignation as a gesture of apology for his terrible mistake.