  Transport properties of Ce$_{0.85}$F0.15SbS$_{2}$ and undoped CeOSbS$_{2}$
under high pressure were investigated experimentally and theoretically.
Electrical resistivity measurements of the Ce$_{0.85}$F0.15SbS$_{2}$ single
crystals were performed under various high pressures using a diamond anvil cell
with boron-doped diamond electrodes. The samples showed the insulator to metal
transition by applying high pressure up to 30-40 GPa. On the other hand, the
undoped CeOSbS$_{2}$ showed almost same transport property with the F-doped
sample under high pressure. The valence state analysis using X-ray
photoelectron spectroscopy revealed a simple valence state of Ce3+ in
Ce$_{0.85}$F0.15SbS$_{2}$ and mixed valence state between Ce3+ and Ce4+ in
undoped CeOSbS$_{2}$. The valence fluctuation in Ce carried out the comparable
transport nature in the both samples. A band calculation suggests that the
undoped CeOSbS$_{2}$ could be metallic under high pressure of 30 GPa in
accordance with the experimental results. A superior thermoelectric property of
power factor in CeOSbS$_{2}$ was estimated under high pressure around 20 GPa in
comparison with that of ambient pressure.
