  Vortex lattice states occurring in noncentrosymmetric superconductors with
the spin-orbit coupling of Rashba type under a magnetic field parallel to the
symmetry plane are examined by assuming the $s$-wave pairing case and in an
approach combining the quasiclassical theory with the Landau level expansion of
the superconducting order parameter. The resulting field-temperature phase
diagrams include not only a discontinuous transition but a continuous crossover
between different vortex lattice structures, and, further, a critical end point
of a structural transition line is found in an intermediate field and at a low
temperature in the present approach. It is pointed out that the strange field
dependence of the vortex lattice structure is a consequence of that of an
anisotropy stemming from the Rashba spin-orbit coupling, and that the critical
end point is related to the helical phase modulation peculiar to these
materials in the ideal Pauli-limited case. Further, calculation results on the
local density of states detectable in STM experiments are also presented.
