As a model for the warped maser disc of NGC 4258,
we have considered m=1 modes in which the density perturbation
is antisymmetric with respect to the equatorial plane.
Solving a linear eigenvalue problem in the 
-plane,
we have found that the velocity field of the mode is dominated by
the vertical component whose amplitude decreases with radius.
This characteristic agrees well with the observed spatial- and
velocity-distribution of the high-velocity maser features of NGC 4258.
Moreover, we have calculated the position-velocity diagram for the disc
with a nonlinear perturbation pattern similar to the linear
warping mode.
We have found that the model diagram reproduces
the observed redward offset of the low-velocity maser features,
if the maser emission arises only from the upper-half (z>0) part
of the disc.
We have also found that the high-velocity maser features
should be shifted redward by an amount somewhat larger than
that [
] expected for
the unperturbed disc.
In our formulation of the eigenvalue problem for warping modes, we have adopted several assumptions for simplicity. The next step should be to formulate the problem with a more realistic disc model in which effects of viscosity, disc self-gravity, and radiation pressure force are taken into account.
acknowledgements: The author thanks H.F. Henrichs for careful reading of the manuscript. The author also thanks the Astronomical Institute 'Anton Pannekoek' for their warm hospitality. This work was partially supported by a grant from Hokkai-Gakuen Educational Foundation.