Marunouchi is a district in central Tokyo, located in Chiyoda ward, the area is between Tokyo station and Kokyo Gaien National Garden which is adjacent to Imperial palace.
Marunouchi with adjacent Otemachi district, it is the CBD, central business district of megalopolis Tokyo, where headquarters of large banks and corporates are located. Therefore there are not so many inhabitants living in Marunouchi, just 17 persons in September 2020.
Marunouchi was developed by Mitsubishi group, all three major companies of Mitsubishi group, Mitsubishi UFJ bank, Mitsubishi Corporation and Mitsubishi Heavy Industries have their head offices in Marunouchi. Therefore it can be said that Marunouchi is a Mitsubishi village.
Due to typical business district, there is no need to due sightseeing in Marunouchi but there are several interesting stores and restaurants in Marunouchi. Then, if you want to see typical Japanese businessmen or businesswomen, better to visit Marunouchi rather than going to Shinjuku.