Herein disclosed are a microwave plasma treating method and a system therefor. A slow wave structure (19) is disposed in the propagation region of microwaves, and the microwaves are thus introduced at a delayed phase velocity into a discharge chamber 10 in which gases are transformed into plasma. Thus, the phase velocity of the microwaves is adjusted to a relatively low velocity, at which charged particles are distributed most densely in the plasma, so that the energy may be efficiently transformed to much more charged particles in the plasma. Thus, the plasma of high density is generated to improve the plasma treating rate.