We investigate the dynamics of the relativistic Sweet-Parker reconnection by means of Relativistic Resistive Magnetohydrodynamic (RRMHD) simulation. It is found that the magnetic energy is preferentially converted to the thermal energy in the diffusion region, resulting the formation of the relativistically hot outflows. The outflow speed is a little smaller than that expected from the non-relativistic theory which predicts the outflow speed is of order the Alfvén speed. The slow outflow speed is the consequence of the increase in the plasma inertia due to the relativistic effects. Since the observed Lorentz factor of the outflow and the reconnection rate are both small, our numerical model suggests that the relativistic Sweet-Parker reconnection is a slow process for the magnetic energy conversion as well as the non-relativistic one.