Marunouchi is a commercial district of Tokyo located in Chiyoda between Tokyo Station and the Imperial Palace. The name, meaning "inside the circle", derives from its location within the palace's outer moat. It is a center of Japan's financial industry, as the country's three largest banks are headquartered there.
A recent revamping of the Marunouchi business district has resulted in some high-end boutiques such as Tiffany's, Gaultier, Sergio Rossi etc. lining the street from the Marunouchi building to Yurakucho, but there is a feeling that it's not quite "there" yet, compared to the repetoires of the Ginza or Omotesando shopping districts.
The Marunouchi Building in front of Tokyo station is your best bet for a decent meal in the Marunouchi area, especially on the weekend when most other buildings are closed. Various restaurants from Italian to Hawaiian to Sushi are situated inside, but the bill may be a bit pricy for a casual lunch.
(reference: Wikipedia, Wikitravel)