Definitely the most famous Japanese cuisine is sushi. I think I do not have to explain it here what sushi is.
Sushi was a fast food, eaten outside in 17th century but now became the one of the luxurious food in Japan. However, the price of sushi is going down than before. Till 1980's, sushi could be only eaten at sushi bar or able to buy at take out store specialized in sushi.
However, sushi can be bought in supermarkets and convenience stores cheaply. Also many izakaya style restaurants and other non-sushi restaurants offer sushi too. Conveyor belt sushi, normally called running sushi in Czech Republic is everywhere in Japan. On the other hand, number of sushi bars are decreasing than before. Only the high end sushi bars remains and local sushi bars are disappearing.
By the way, sushi combo is usually classified as Matsu(Pine), Take(Bamboo) and Ume(Japanese apricot). Matsu is the most expensive combo(using expensive ingredients) and Ume, the cheapest. Then, usually Matsu has more pieces of sushi than Take.