  We analyze the dynamical expansion of the HII region and outer
photodissociation region (PDR) around a massive star by solving the UV and FUV
radiation transfer and the thermal and chemical processes in a time-dependent
hydrodynamics code. We focus on the physical structure of the shell swept up by
the shock front (SF) preceding the ionization front (IF). After the IF reaches
the initial Stromgren radius, the SF emerges in front of the IF and the
geometrically thin shell bounded with the IF and the SF is formed. The gas
density inside the shell is about 10-100 times as high as the ambient gas
density. Initially the dissociation fronts expands faster than IF and the PDR
is formed outside the HII region. Thereafter the IF and SF gradually overtakes
the proceeding dissociation fronts (DFs), and eventually DFs are taken in the
shell. The chemical composition within the shell is initially atomic, but
hydrogen and carbon monoxide molecules are gradually formed. This is partly
because the IF and SF overtake DFs and SF enters the molecular region, and
partly because the reformation timescales of the molecules become shorter than
the dynamical timescale. The gas shell becomes dominated by the molecular gas
by the time of gravitational fragmentation, which agrees with some recent
observations. A simple estimation of star formation rate in the shell can
provide a significant star formation rate in our galaxy.
