The concept of gauge transformation in soliton theory is introduced. It is shown that three different eigenvalue problems in 2x2 matrix form are connected by the gauge transformations. As an application of the theory, the interrelations among the nonlinear Schrodinger equation, the derivative nonlinear Schrodinger equation, isotropic Heisenberg equation and a new integrable equation are studied. © 1983, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.