  In this paper, we solve the massive scalar field in the Reissner-Nordstr\"om
spacetime. The scalar field in the Reissner-Nordstr\"om spacetime has both
bound states and scattering states. For bound states, we solve the bound-state
wave function and the eigenvalue spectrum. For scattering states, we solve the
scattering wave function and give an explicit expression for scattering phase
shift by the integral equation method. Especially, we introduce the tortoise
coordinate for the Reissner-Nordstr\"om spacetime.
