  We investigate the evolution of molecular abundance in quiescent
protoplanetary disks which are presumed to be around weak-line T Tauri stars.
In the region of surface density less than $10^2$ g cm$^{-2}$ (distance from
the star $\gtrsim 10$ AU in the minimum- mass solar nebula), cosmic rays are
barely attenuated even in the midplane of the disk and produce chemically
active ions such as He$^+$ and H$_{3}^+$. Through reactions with these ions CO
and N$_2$ are finally transformed into CO$_2$, NH$_3$, and HCN. In the region
where the temperature is low enough for these products to freeze onto grains,
considerable amount of carbon and nitrogen is locked up in the ice mantle and
is depleted from the gas phase in a time scale $\lesssim 3\times 10^6$ yr.
Oxidized (CO$_2$) ice and reduced (NH$_3$ and hydrocarbon) ice naturally
coexist in this part of the disk. The molecular abundance both in the gas phase
and in ice mantle varies significantly with the distance from the central star.
