We have developed the apparatus that can measure the three-dimensional distribution of microwave electric field intensity in electron cyclotron resonance (ECR) plasmas to investigate production and control of ECR plasmas. The relationship between the plasma properties of ECR plasmas and the microwave electric field intensity in plasmas is studied. We have confirmed that the pattern of the radial distribution of the ion saturation current at the electrode is the same as that of the microwave electric field intensity at the ECR zone. If the distribution of microwave electric field intensity at the ECR zone is uniform, the distribution of plasma density on the electrode becomes uniform, even if the distribution of microwave electric field intensity of the other zone is not uniform. Therefore, in order to obtain the optimum distribution of plasma density on the electrode, the distribution of microwave electric held intensity at the ECR zone must be controlled.