  In a disk around DM Tau, previous observation of 13CO (J=2-1 and 1-0
transitions) derived the 13CO gas temperature of \sim 13-20K, which is lower
than the sublimation temperature of CO (20 K). We argue that the existence of
such cold CO can be explained by a vertical mixing of disk material. As the gas
is transported from a warm layer to a cold layer, CO is depleted onto dust
grains with a timescale of \sim 10^3 yr. Because of the steep temperature
gradient in the vertical direction, an observable amount of CO is still in the
gas phase when the fluid parcel reaches the layer of \sim 13 K. Apparent
temperature of CO decreases as the maximum grain size increases from
micron-size to mm-size.
