For pt.II see ibid., vol.12, p.1105 (1979). The dynamical process by which the steady propagation of a coherent light pulse of long width takes place in a dielectric medium is studied. In the absence of direct interaction between atomic dipoles, the nonlinear polariton is unstable against a small perturbation and develops self-modulation of its envelope. Nonlinear Schrodinger equations describing this self-modulation are derived for the two cases where the carrier wave frequency lies outside and inside the polariton gap. It is shown that an arbitrary incoming pulse of long width outside the polariton gap evolves as composite pulse of multiple peak structure, which is regarded as a bound state of the steady pulses obtained in a previous paper. The evolution process of pulse inside the polariton gap and the effect of direct interaction between atoms are also discussed.