The shear band and associated stress relaxation classically known as the Portevin-Le Chatelie effect is considered based on physical mesomechanics, a recent field theory on plastic deformation and fracture. Using the mathematical analogy to the Maxwell's electromagnetic theory, the field theoretical meanings of various properties of plastic deformation are considered. For more intuitive understanding, a simple, two-dimensional spring-mass model is introduced. Along these considerations, interpretations are given to various experimental observations, in which the shear band is detected as an optical interferometric band structure. The results of this investigation are consistent with our previous identification of the optical band structure as a current associated with symmetry.