  We propose a new method of the $\beta$-$\gamma$ constraint for quadrupole
deformation in antisymmetrized molecular dynamics (AMD) to describe various
cluster and shell-model structures in the ground and excited states of light
nuclei. We apply this method to N=6 isotones, $^{10}$Be, $^{12}$C, $^{9}$Li,
and $^{11}$B, and find various structures as functions of the deformation
parameters, $\beta$ and $\gamma$. In these nuclei, shell-model-like structures
appear in the small $\beta$ region, while cluster structures develop well in
the large $\beta$ region where various geometric configurations of clusters are
obtained depending on the $\gamma$ parameter. For $^{10}$Be and $^{12}$C, we
superpose the basis AMD wave functions obtained by the $\beta$-$\gamma$
constraint method to calculate energy spectra, and prove the advantages of the
present method of the two-dimensional $\beta$-$\gamma$ constraint in the
framework of AMD.
