The covariant string field theory is presented in its full detail for the closed-bosonic-string case. The properties of the interaction vertex of three closed strings are fully clarified. We thus establish the nilpotency of the full nonlinear Becchi-Rouet-Stora (BRS) transformation as well as the BRS invariance of our gauged-fixed action. This, on the other hand, also establishes the gauge invariance of our gauge-unfixed action and the group law of the gauge transformations, which were reported previously. The general N-point amplitudes for arbitrary external closed-string states are computed explicitly at the tree level. In particular, when the external states are on shell and physical, we prove that the amplitudes become independent of the choice of the external string-length parameters and correctly reproduce the usual dual amplitudes.