  The contraction processes of metal-free molecular clouds of starlike mass (or
cloud cores) are investigated. We calculate radiative transfer of the H_2 lines
and examine quasi-static contraction with radiative cooling. Comparing two
time-scales, the free-fall time t_ff and the time-scale of quasi-static
contraction t_qsc (nearly equal to t_cool, the cooling time) of these cores, we
find that the ratio of the two time-scales t_ff/t_qsc, i.e., the efficiency of
cooling, becomes larger with contraction even under the existence of cold and
opaque envelopes. In particular, for fragments of primordial filamentary
clouds, for which t_ff is nearly equal to t_qsc at the fragmentation epoch,
they collapse dynamically in the free-fall time-scale. This efficiency of
cooling is unique to line cooling.
