  Results of an SiO maser survey for the late-type stars selected by the IRTS
(Infrared Telescope in Space) are presented. We have detected SiO J=1-0, v=1
and/or v=2 lines in 27 stars out of 59 stars. The maser intensity increases
with the depth of the H2O absorption in the infrared spectra and redness of the
2.2 and 12 micron color. The column densities of the water vapor in the target
stars are estimated from the depth of the water absorption in the IRTS spectra.
We found that the SiO maser was detected mostly in the stars with the column
density of water vapor higher than 3x10^19-3x10^20 cm^-2. We further estimate
the density of hydrogen molecules in the outer atmosphere corresponding to
these column densities, obtaining 10^9 - 10^10 cm^-3 as a lower limit. These
values are roughly in agreement with the critical hydrogen density predicted by
models for the excitation of the SiO masers. It is possible that the SiO masers
are excited in clumps with even higher than this density. The present results
provide useful information on the understanding of the physical conditions of
the outer atmospheres in late-type stars.
