What is the dominant gas phase in interstellar space? What is the
dominant energy source for such gaseous phase? These are two of the
biggest problems that left in interstellar physics. We observed the
Cygnus X region through [CII] 158 micron line and detected intense
diffuse [CII] emission. The [CII] line is the dominant coolant of
interstellar gas and thus one of powerful proves to study physical
properties of interstellar space. However, wether the dominant emitter
of the [CII] line is neutral gas or ionized gas is unclear. Thus,
interpretation of the observation has large ambiguity. In order to
reveal the physical properties of the dominant interstellar gas phase
which is traced by the [CII] line, we propose far-infrared
spectroscopic observations using the LWS full grating scan toward the
Cyg-OB2 association. Multi-line spectroscopy by LWS will reveal
physical properties of the dominant gas phase with least ambiguity.