In this book, we start with radiation from a source and explain the equivalent theorem for electromagnetic waves. After that, the equivalent mathematical analysis model is explained in relation to the analysis of differential equations. Specifically, the solution of the electromagnetic scattering problem is explained in terms of structures conforming to the orthogonal coordinate system. The goal of this book is to understand the physical phenomena of field problems by using knowledge of mathematics. Specifically, the differential equations are solved by using the knowledge of Fourier series, Fourier transform, complex functions and so on, in addition to physical conditions. In recent years, there are many cases in which the field boundary value problems are solved by general-purpose field simulators, even for complex shapes. However, it is difficult to understand the essence and universality of physical phenomena by only simulating the field distribution in animation. Returning to the few canonical field problems treated in this book, we believe that understanding the universal behavior of wave phenomena and the equivalent relationship between analytical and real models are useful for understanding and improving the accuracy of approximate solutions of complex phenomena.