Hamburger is popular in Japan too. Of course there are fast food chains which you can see in Europe but there are also Japanese local chains too.
Sasebo, located in north west of Kyushu island, is the second largest city in Nagasaki prefecture next to prefectural capital Nagasaki. Sasebo is a major port, especially naval port both for Japan and US navy.
Then, due to a lot of US navy sailors are based in Sasebo, Sasebo is famous for Sasebo burger.
On the other hand, Hakodate, located in the northern island Hokkaido is also famous for hamburgers. Hakodate is also a major port, used to be most populated city in Hokkaido, now is third populated city behind Sapporo and Asahikawa but it is one of the major tourist destinations in Hokkaido. Sasebo burger are provided by several local hamburger shops but hamburger in Hakodate district is dominated by Lucky Pierrot which has dozen of stores throughout Hakodate district.
If you travel Japan for more than a week, you should try the western foods which are adapted to Japanese tastes.