The large scale observation of our Galactic plane with a balloon
 telescope in the [C II] 158 mm line shows that our Galactic
 center has lower [C II]/FIR continuum intensity ratio than
 that in general Galactic plane region. The line/continuum
 ratio has rather flat distribution over wide area of the
 general Galactic plane. Thus, this large scale (500pc)
 defect of [C II] line intensity is not local effect,
 but is important for Galactic structure.
 We have proposed two possible sources of the [C II] defect:
 1) molecular self-shielding in dense molecular clouds in
 the Galactic center region; 2) added heating of grains by
 late-type stars in the bulge. These two decrease formation of
 C+ ions, which emit [C II] line, and/or excitation of the
 ions to upper level of the transition.
 We propose observation of [C II] intensity along the major
 axis of M31, whose center has larger bulge and lesser
 molecular gas than the Galactic center. The center with
 different physical conditions from our Galactic Center
 should give answer for this problem.