Mass loss of AGB stars is an important process for the evolution
 of matter in the Galaxy.  However, there are a number of unknowns
 involved in the mass loss process.  The largest uncertainties
 reside in the formation process and optical properties of the dust
 grains.  Model analysis of infrared observations often suggests
 a low temperature of silicate dust formation compared to
 theoretical predictions, indicating a reconsideration of dust
 formation and mass loss process.  On the other hand,
 interpretation of observations requires the photon absorption
 efficiency of silicate dust to be much higher than that of any
 known silicate existing on the earth ("dirty silicate model").
 The amount of increase of the absorptivity is often arbitrarily
 assumed.  Both problems interrelate with each other and are
 difficult to be investigated separately by analyses solely based
 on an IR spectrum of the dust shell.  We propose to elucidate
 these problems by observing optically thin dust shell around
 oxygen-rich long-period variables periodically by ISO.  Periodic
 observations from near to far infrared will provide the
 information of the reaction of dust shell to different incident
 radiations from the photosphere.  We can determine the change of
 the dust temperature and the amount of dust grains separately
 owing to the simple nature of optically thin dust shell from
 the observations at different epochs.  From these results and
 the energy balance consideration we can derive the absorptivity
 of silicate dust quantitatively and identify the dust forming
 region.  ISO will provide a unique opportunity to make such
 periodic observations of the wide IR region with stable accuracy,
 which are essential for this kind of study.  The results will
 lead to a better understanding of the late stellar evolution and
 the circulation of the matter in the Galaxy.