  The cold dark matter (CDM) cosmology, which is the standard theory of the
structure formation in the universe, predicts that the outer density profile of
dark matter halos decreases with the cube of distance from the center. However,
so far not much effort has examined this hypothesis. In the halo of the
Andromeda galaxy (M31), large-scale stellar structures detected by the recent
observations provide a potentially suitable window to investigate the
mass--density distribution of the dark matter halo. We explore the density
structure of the dark matter halo in M31 using an N-body simulation of the
interaction between an accreting satellite galaxy and M31. To reproduce the
Andromeda Giant Southern Stream and the stellar shells at the east and west
sides of M31, we find the sufficient condition for the power-law index $\alpha$
of the outer density distribution of the dark matter halo. The best-fit
parameter is $\alpha=-3.7$, which is steeper than the CDM prediction.
