  Ordering of the frustrated classical Heisenberg model on the
triangular-lattice with an incommensurate spiral spin structure is studied
under magnetic fields by means of a mean-field analysis and a Monte Carlo
simulation. Several types of multiple-q states including the "skyrmion-lattice"
state is observed in addition to the standard single-q state. In contrast to
the Dzyaloshinskii-Moriya interaction driven system, the present model allows
both skyrmions and anti-skyrmions, together with a new thermodynamic phase
where skyrmion and anti-skyrmion lattices form a domain state.
