Because the refractive index of unmagnetized uniform plasmas is approximately expressed as n =(1-ω2p0 / ω2)1/2 and the electron velocity v < c in the limit of relativity effect, the phase velocity of the EM wave becomes vp =c / n > c due to n < 1 and the Cherenkov condition, nv / c > 1, cannot be satisfied, where ω (ωp0) is the EM wave (plasma) frequncy and c is the light velocity in vacuum.