We propose to make multi-line spectroscopic observations of the
selected areas in the Galactic center region on a large scale (+- 5
degrees) to reveal the origins of the conspicuous [CII] 158 micron
line deficit in the Galactic center (GC).
Our balloon observations revealed that the [CII] line is
surprisingly weak toward the GC, and the ratio of the [CII] line
emission to far-infrared continuum (I_[CII]/I_FIR) shows a large dip
at the central several hundred parsecs of the GC. The results indicate
the low abundance of the C+ ions in the GC area. We hypothesized that
this low  abundance is mainly due to soft UV radiation with fewer
C-ionizing photons. This soft radiation field, together with the
pervasively high molecular gas density, makes the molecular
self-shielding more effective, and raises temperature of molecular gas
at the C+/C/CO transition zone. Hence our hypothesis can naturally
explain not only the weak [CII] line but also the pervasive
existence of warm molecular clouds in the GC.
Since our observed [CII] line alone is not sufficient to determine
the physical conditions and to check our hypothesis, we propose here
multi-line spectroscopic observations. The multi-line spectroscopy by
the ISO will not only reveal the origins of this [CII] line deficit
and but also to reveal the energy budget of  extraordinary
interstellar clouds in the GC.
We are now making various observations of Photo Dissociation Regions
(PDRs) using the ISO. The current program is a natural extension of
our systematic study of PDRs. Hence, in this proposal, more time is
being requested for TONAKA.WDISM_1 and TONAKA.WDISM_2.