The dynamic behavior of soliton solutions in nonintegrable extended Klein–Gordon systems as a continuum containing a dissipation term and an external force term, expressed by φxx−φtt−F(φ)=Gφt−JB, is investigated geometrically in a state plane by transforming the equation into three basic equations, each of which is associated with the derivative with respect to x, t, and φ, respectively. The initial and the boundary conditions are imposed so that the solution approaches asymptotically the stationary solitary-wave solution as ‖x‖ and ‖t‖ approach infinity. In the above treatment, the waves consisting of the pair of φx and φt are divided into two components, the traveling wave component, V(x,t), and the others. The state plane is then constructed by the coordinates consisting of V(x,t) and φ. The traveling wave component is defined with φT by introducing nonlinear coordinates Ξ(x,t)=X(x,t)−uT(x,t) instead of the linear coordinates ξ=x−ut, and travels with a constant velocity of u in the coordinates. Their properties are investigated for various cases of wave interactions. The method for analyzing soliton interactions is described in detail. It is first shown that the solutions to the basic equations presented are in agreement with the well-known solutions for two soliton interactions in pure sine–Gordon system. Next, as an example, the analytical method is applied to the extended sine–Gordon system, the behavior of the local distortions produced during the soliton–antisoliton interaction because of existence of the moving singularities is described, and details of the mechanism and properties of them are clarified. It is finally shown that the additional dissipation term proportional to φxxt acts to smooth the distortions resulting from the disappearance of the moving singularities.