  The triaxial deformation in $^{10}$Be is investigated using a microscopic
$\alpha+\alpha+n+n$ model. The states of two valence neutrons are classified
based on the molecular-orbit (MO) model, and the $\pi$-orbit is introduced
about the axis connecting the two $\alpha$-clusters for the description of the
rotational bands. There appear two rotational bands comprised mainly of $K^\pi
= 0^+$ and $K^\pi = 2^+$, respectively, at low excitation energy, where the two
valence neutrons occupy $K^\pi = 3/2^-$ or $K^\pi = 1/2^-$ orbits. The
triaxiality and the $K$-mixing are discussed in connection to the molecular
structure, particularly, to the spin-orbit splitting. The extent of the
triaxial deformation is evaluated in terms of the electro-magnetic transition
matrix elements (Davydov-Filippov model, Q-invariant model), and density
distribution in the intrinsic frame. The obtained values turned out to be
$\gamma = 15^o \sim 20^o$.
