In 1989, the Japanese Ministry of Education had made it a part of the national physical education program for all third and fourth graders to ride unicycles. So almost everyone in the country has ridden a unicycle at some point.
Many schools offer unicycling classes and encourage kids to join unicycling clubs, not just because unicycles are fun but also because they help develop a sense of balance. Almost any elementary-school-age kid can learn to ride in just a week, and can become an expert after a few months of practice.
Each year, the Japan Unicycling Association has held an official contest in the Kansai area, where unicyclists are ranked from levels 1 through 10 according to their skills.
I think it's pretty fascinating that something like a unicycle, considered a specialty vehicle in America, is so widely acknowledged in Japan.