This book deals with Korea nationalism under Japanese colony. Book title is 'the age of nationalism' because 'nationalism' is most significant keyword influenced Korea history in early 20th century. In world history, the age of nationalism, of course, was 19th century. Nationalism was born during Civil War and in the process of establishing nations, and led the trend of age. However, in Korea, nationalism was born in early 20th century of crisis of ruining country and since then, it became most significant keyword in Korea history. Nationalism under colonial situation was most significant theoretical evidence in the process of intending the construction of nation, independence of nation. Also, after liberation, it was still significant philosophy because of two challenges, construction of country and reunification. For these reasons, the author regards Korea in 20th century as 'the age of nationalism'.