  The edge-on Sc galaxy NGC 5907 has been observed in the 12CO(1-0)-line
emission using the Nobeyama 45-m telescope. The radial density distribution
between 2 and 13 kpc is well represented by a superposition of an
exponential-law disk of scale radius 3.5 kpc and a ring of 7 kpc radius.
However, no concentration of gas has been observed in the central 2 kpc. The
radial distribution and position-velocity diagram are compared in detail with
those obtained for HI. We found that the molecular gas dominates in the central
5 kpc region, while HI does in the outer region, clearly separated by a
``molecular front'' at 5 kpc radius. This molecular front will be the place
where the phase change from HI to molecular hydrogen, or vice versa, is taking
place in a galactic scale. The central deficiency of the molecular gas is
exceptional for a late type galaxy, which was found for the first time among Sc
galaxies, and the second case after the Sb galaxy M31. We argue for the
possibility of destruction of the nuclear gas disk by a merger of another
galaxy, and discuss its relation to the outer warping disk.

Astrophysics