According to him, Finland will be a very strong link in the eastern flank of NATO.
"We know what it is known for – strong aviation, strong artillery, large and very well-trained reserves," RIA Novosti quoted Rau as saying.
The Polish minister added that Finland's accession to NATO is "a very good acquisition and strengthening."
Earlier, Finland officially became the 31st member of the organization.
Russian Deputy Foreign Minister Sergei Ryabkov said that the Russian side will respond to Finland's accession to the North Atlantic Alliance.
According to him, Moscow will calmly make public, when the time comes, what Russia will do in response.