Injecting a high power microwave into the CT-6B tokamak plasma, we performed the experiments on the pre-ionization of the tokamak discharge at the fundamental and the second harmonic resonance of the electron cyclotron wave. The experiments verified that the microwave pre-ionization is very effective for the reduction of the loop voltage and the magnetic flux in the early stage and suppression of the production of runaway electrons in the quasi-steady stage. The effect of the microwave pre-ionization with the fundamental cyclotron resonance is better than that with the second one.