  We show a possible way to measure the column density of free electrons along
the light path, the so-called Dispersion Measure (DM), from the early $[\sim
415 (\nu/1 {\rm GHz})^{-2} ({\rm DM}/10^{5} {\rm pc} {\rm cm}^{-3}) {\rm s}]$
radio afterglows of the gamma-ray bursts. We find that the proposed Square
Kilometer Array can detect bright radio afterglows around the time $\sim
10^{3}(\nu/160 {\rm MHz})^{-2}$ s to measure the intergalactic DM ($\simg 6000$
pc cm$^{-3}$ at redshift $z>6$) up to $z\sim 30$, from which we can determine
the reionization history of the universe and identify the missing warm-hot
baryons. At low $z$, DM in the host galaxy may reach $\sim 10^{5}$ pc cm$^{-3}$
depending on the burst environment, which may be probed by the current
detectors. Free-free absorption and diffractive scattering may also affect the
radio emission in a high density.
