The density ratio of negative ions to positive ions alpha in a SF6/Ar double plasma, O-2/Ar, CF4/Ar and C4F8/Ar electron cyclotron resonance (ECR) plasmas were estimated using the propagation characteristics of the ion acoustic waves (fast mode). Here, the phase velocity of the fast mode in a double plasma and an ECR plasma were measured with the interferometric method and the time of flight method, respectively. As a result, it was found that alpha was more than 0.9 in the SF6/Ar double plasma, and less than 0.5, 0.3 and 0.4 in the O-2/Ar, CF4/Ar and C4F8/Ar ECR plasma, respectively. Furthermore, it was observed that a has a large value in the lower gas mixture rate region in the case of ECR plasmas.