  In this study, the superconducting gap symmetry of LaO$_{0.5}$F$_{0.5}$BiSSe
is elucidated through specific heat measurements with one single crystal of
LaO$_{0.5}$F$_{0.5}$BiSSe. A clear jump is observed at 3.95 K in
LaO$_{0.5}$F$_{0.5}$BiSSe, suggesting bulk superconductivity. From specific
heat measurements, $\Delta C_\mathrm{e}/\gamma T_\mathrm{c}$ and
2$\Delta$(0)/$k_\mathrm{B}T_\mathrm{c}$ are obtained as 2.31 and 4.5. These
values indicate strong electron-phonon coupling superconductivity. The
temperature ($T$) and magnetic-field ($H$) dependence of the electronic
specific heat $C_\mathrm{e}(T, H)$ suggests that superconductivity is fully
gapped. Our handmade system successfully elucidates superconducting gap
symmetry of the BiS$_2$-based compound with a small single crystal.
