It has been more than one hundred years since the first elementary particle was discovered. Physicists have contributed to establishing the theoretical frame to understand the structure of matter and properties of forces in nature. As approaching the core of nature, we also observe the relations between different regimes of physics, such as properties between elementary particles and the beginning of the universe. The common aspects of physical fields provide a profound way to generalize the theories. The theory of relativity gave us a unified perspective toward classical- and electro- dynamics. Also the description of a small scale, quantum mechanics, has been successful under the philosophy of physics. Both theories, relativity and quantum, are merged as the theory of quantum fields, which facilitates more accurate and extensive insight to the unification scheme. On the other hand, nuclear physics focuses on the structure of matter, and the description involves numerous parts of physics. This book attempts to introduce the nuclear physics in a more systematic way, particularly with the field-theoretic approach.