  We formulate the theory for a self-propelled domain in an excitable
reaction-diffusion system in two dimensions where the domain deforms from a
circular shape when the propagation velocity is increased. In the singular
limit where the width of the domain boundary is infinitesimally thin, we derive
a set of equations of motion for the center of gravity and two fundamental
deformation modes. The deformed shapes of a steadily propagating domain are
obtained. The set of time-evolution equations exhibits a bifurcation from a
straight motion to a circular motion by changing the system parameters.
