Located on the second floor of the Akihabara UDX building, Kamugen is popular spot to get soba but its giant rice balls are also a favorite for takeout orders. They have 5 different types of rice balls such as the tuna and mayonnaise rice ball and shrimp ahijo rice ball. But the roast beef rice ball is the most popular.
The roast beef rice ball has a boiled egg in the middle which is encased with rice the whole thing is wrapped in tasty roast beef. Be sure to get there early to try the roast beef rice balls since they usually sell out early.
(Source: Japan Travel by NAVITIME)