A novel long linear-type microwave plasma source using a variably-reduced-height rectangular wave-guide as the plasma reactor has been developed. Microwave power is fed from the both sides of the wave-guide and is coupled into plasma through a long slot cut on the broad side of the wave-guide. The reduced height of the wave-guide is variable in order to control the coupling between microwave and plasma so that the plasma is able to attain better uniformity when extending the length of the linear-type plasma source.