Negative ion density in low-pressure (25 mtorr) high-density inductively coupled plasmas of SF6/Ar NF3/Ar CF4/Ar and C4F8/Ar mixture gases was measured using the laser photodetachment method combined with the millimeter-wave resonance technique, Electron density in the active glow as well as the decay rate of the electron density in the afterglow were also measured. It was shown that, at a molecular-gas mixing ratio as low as 5-10% and at an electron density as high as similar to 10(11) cm the negative ion density is comparable to or higher than the electron density. flow the electron attachment rate in the plasma varies with the dissociation degree of the parent gas molecules is discussed on the basis of measured data as well as in comparison with previous reports for low-density plasmas.