In this paper, we discuss an optimal shape of the grounded part of a snake robot with passive wheels for jumping. First, we derive a relationship between constraint forces for the wheels and necessary friction forces, and propose a constraint force ellipse. Next, we introduce an evaluation index for the optimal shape of the grounded part (base part) of a snake robot using the constraint force ellipse. Finally, in order to demonstrate the validity of the optimal shape that minimizes the evolution index, simulations have been carried out.