  Monte Carlo simulation study of a classical spin model with
Dzylosinskii-Moriya interaction and the spin anisotropy under the magnetic
field is presented. We found a rich phase diagram containing the multiple spin
spiral (or skyrme crystal) phases of square, rectangular, and hexagonal
symmetries in addition to the spiral spin state. The Hall conductivity
$\sigma_{xy}$ is calculated within the $sd$ model for each of the phases. While
$\sigma_{xy}$ is zero in the absence of external magnetic field, applying a
field strength $H$ larger than a threshold value $H_c$ leads to the
simultaneous onset of nonzero chirality and Hall conductivity. We find $H_c =
0$ for the multiple spin spiral states, but $H_c > 0$ for a single spin spiral
state regardless of the field orientation. Relevance of the present results to
MnSi is discussed.
