Introduces basic matters and theorems related to internal force, deformation, stress, and strain of elastic one-dimensional member. For solid industrial materials, we introduce the concept of sectional forces (axial force, shear force, bending moment), which are internal forces, and stress and strain, and derive the constitutive law that is the relationship between internal force or stress and deformation or strain. Using them, we introduce simple truss structures and beam bending theory.