  We extend the formulation of the spin wave theory for the Kondo lattice
model, which was mainly used for the ferromagnetic metallic state, to general
magnetic orders including complex noncollinear and noncoplanar orders. The 1/S
expansion is reformulated in the matrix form depending on the size of the
magnetic unit cell. The noncollinearity and noncoplanarity of the localized
moments are properly taken into account by the matrix elements of the
para-unitary matrix used in the diagonalization of the Bogoliubov-de Gennes
type Hamiltonian for magnons. We apply the formulation within the linear spin
wave approximation to a typical noncollinear case, the $120^{\circ}$ N{\'e}el
order on a triangular lattice at half filling. We calculate the magnon
excitation spectrum and the quantum correction to the magnitude of ordered
moments as functions of the strength of the Hund's-rule coupling, $J_{\rm
H}/t$. We find that the magnon excitation shows softening at $J_{\rm H}/t
\simeq 2.9$, which indicates that the $120^{\circ}$ order is destabilized for
smaller $J_{\rm H}/t$. On the other hand, we show that the $120^{\circ}$ order
is stable in the entire range of $J_{\rm H}/t \gtrsim 2.9$, and, in the limit
of $J_{\rm H}/t \to \infty$, the form of the spin wave spectrum approaches that
for the antiferromagnetic Heisenberg model, while the bandwidth is proportional
to $t^2/J_{\rm H}$. The reduction of the ordered moment is smaller than that
for the spin-only model, except in the vicinity of the softening.
