We study the spiral wave in an unbounded excitable medium from the wave front interaction model derived by Zykov in 2009. This model consists of two systems of ordinary differential equations that describe the wave front and the wave back, respectively. First, we derive some properties of the back by the shooting argument and the comparison principle. Next we show the global existence of the solution of the back. Then we study its asymptotic behavior at infinity. Finally, we prove the uniqueness of the solution.