  We re-examine the thermal evolution of the postshock layer in primordial gas
clouds. Comparing the time scales, we find that the evolutionary paths of
postshock regions in primordial gas clouds can be basically understood in terms
of the diagram drawn in the ionization degree vs temperature plane. The results
obtained from the diagram are independent of the density in the case that we do
not include photodissociation and photoionization. We also argue that the
diagram is not only relevant to the case of the steady postshock flow, but also
to the isochorically cooling gas.
