  Dynamical Chern-Simons gravity has an interesting feature that the parity
violating term exists, and the coupling is determined by a dynamical scalar
field. When the spacetime has spherical symmetry, the parity violating term
vanishes, and then the metric of the Schwarzschild spacetime with vanishing
scalar field is an exact solution of dynamical Chern-Simons gravity. The effect
of the Chern-Simons coupling appears in the study of perturbation around the
Schwarzschild spacetime. Due to the parity violating term, the odd parity
metric perturbation and the perturbed scalar field are coupled, and the
perturbed field equations take the form of the coupled system of the
Schr\"odinger equations. We prove linear mode stability for a generic massive
scalar.
