  We derive 5D N=1 superspace action including the radion superfield. The
radion is treated as a dynamical field and identified as a solution of the
equation of motion even in the presence of the radius stabilization mechanism.
Our derivation is systematic and based on the superconformal formulation of 5D
supergravity. We can read off the couplings of the dynamical radion superfield
to the matter superfields from our result. The correct radion mass can be
obtained by calculating the radion potential from our superspace action.
