  We investigate the electronic transport property of lateral heterojunctions
of semiconducting and metallic transition-metal dichalcogenide monolayers,
MoSe$_2$ and NbSe$_2$, respectively. We calculate the electronic transmission
probability by using a multi-orbital tight-binding model based on the
first-principles band structure. The transmission probability depends on the
spin and valley degrees of freedom. This dependence qualitatively changes by
the interface structure. The heterostructure with a zig-zag interface preserves
the spin and the valley of electron in the transmission process. On the other
hand, the armchair interface enables conduction electrons to transmit with
changing the valley and increases the conductance in hole-doped junctions due
to the valley-flip transmission. We also discuss the spin and valley
polarizations of electronic current in the heterojunctions.
