The book deals with quantum field theory which is the language of the modern physics of elementary particles. Written based on university lectures given by the author, the book provides treatments and technical details of quantum field theory, which will be particularly useful for students. The book starts with the quantization of the most important kind of free fields (the scalar, the spin-1/2 and the photon fields). It is then followed by a detailed account of the symmetry properties of a field theory and a discussion on global and local symmetries and the spontaneous breaking of symmetries. Other topics discussed include the perturbation theory, one-loop effects for quantum electrodynamics, and renormalization properties.