A macroscopic system consists of a tremendous number of microscopic atoms and molecules. In thermal equilibrium the state of such a system is uniquely defined, despite the fact that the microscopic particles behave quite randomly. This observation gives rise to the fundamental law of the statistical physics; it allows entropy to be defined and a framework for the theory to be constructed. The first part of this book explains the essence of statistical physics without going into details such as Liouville’s theorem or ergodic theorem, which are difficult for beginners and unnecessary for actual application of the statistical mechanics. In the second part, statistical mechanics ís applied to various systems which look different but have the same mathematical structure. In this way readers can deepen their understanding of statistical physics.