The negative ion density in a SF6/Ar double plasma is estimated. Here, the density ratio of negative ions to positive ions or is evaluated from the reduction rates of the ion and electron saturation current. Furthermore, the negative ion density obtained with this method is confirmed to agree with that calculated from the measured phase Velocity of the ion acoustic wave (fast mode) when alpha < 0.6, where the positive and negative ion mass are obtained from the spectrum analysis with a quadruple mass spectrometer (QMS) system. The negative ion density in fluorocarbon ECR plasma is also estimated by means of the Langmuir probe method. It is found that alpha in fluorocarbon ECR plasma is less than 0.5.