  It is demonstrated that a {\veck}-dependence of the hybridization matrix
element between $f$- and conduction electrons can give rise to an anisotropic
hybridization gap of heavy fermions if the filling of electrons corresponds to
that of the band insulator. The most interesting case occurs when the
hybridization vanishes along some symmetry axis of the crystal reflecting a
particular symmetry of the crystal field. The results of a model calculation
are consistent with wide range of anomalous properties observed in CeNiSn and
its isostructural compounds, the anisotropic semiconductor of heavy fermions.
In particular, highly sensitive effect of impurity scattering on the residual
density of states for zero energy excitation and the anisotropic temperature
dependence of the resistivity are well explained. It is also discussed that a
weak semimetallic behavior arises through the weak $\veck$-dependence of the
$f$-electron self-energy $\Sigma_{f}(\veck,0)$.
