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.