  We investigate the superconducting-gap anisotropy in one of the recently
discovered BiS$_2$-based superconductors, NdO$_{0.71}$F$_{0.29}$BiS$_2$ ($T_c$
$\sim$ 5 K), using laser-based angle-resolved photoemission spectroscopy.
Whereas the previously discovered high-$T_c$ superconductors such as copper
oxides and iron-based superconductors, which are believed to have
unconventional superconducting mechanisms, have $3d$ electrons in their
conduction bands, the conduction band of BiS$_2$-based superconductors mainly
consists of Bi 6$p$ electrons, and hence the conventional superconducting
mechanism might be expected. Contrary to this expectation, we observe a
strongly anisotropic superconducting gap. This result strongly suggests that
the pairing mechanism for NdO$_{0.71}$F$_{0.29}$BiS$_2$ is unconventional one
and we attribute the observed anisotropy to competitive or cooperative multiple
paring interactions.
