We know that ordinary matter is made of atoms. An atom consists of the atomic nucleus and the electrons. The atomic nucleus is made of a number of the protons and the neutrons. The proton and the neutron are further made of two kinds of quarks, u and d. Therefore the fundamental building blocks of ordinary matter are the electron and two kinds of quarks, u and d. The standard model, which gives comprehensive description of current understanding of the elementary particle phenomena, however, tells us that the number of species of quarks is six. The additional quarks are called s, c, b and t. The reason why we do not find them in the ordinary matter is that they are unstable in the usual environment. Similarly, the electron belongs to a family of six members called leptons. Three types of the neutrinos are included among these six. Another important ingredient of the standard model is the fundamental interactions. Three kinds of interactions act on the quarks and leptons. The strong interaction is described by Quantum Chromodynamics (QCD) and the electro-magnetic and weak interactions by the Weinberg–Salam–Glashow theory in a unified manner. All of them belong to a special type of the field theory called gauge theory. The standard model was established in 1970’s. It was triggered by the development of the studies of gauge theories. In particular, it was proved that the generalized gauge theory is renormalizable. This opened the possibility that all the interactions of the elementary particle can be described by the quantum field theory without the difficulty of divergence. Before this time, such description was possible only for the electro-magnetic interaction. The discovery of the new flavors made in 1970’s played an important role in the establishment of the standard model. In particular, the τ -lepton and cand