  Resistivities of single-crystalline as well as poly-crystalline samples of
CeOBiS2 without fluorine doping were measured at temperatures down to 0.13 K,
and were compared with those of poly-crystalline LaOBiS2 and PrOBiS2. Both
poly-crystalline and single-crystalline CeOBiS2 exhibited zero resistivity
below 1.2 K while poly-crystalline LaOBiS2 and PrOBiS2 did not show zero
resistivity down to 0.13 K. Superconducting transition temperature of CeOBiS2
was reduced by increasing the applied current density. The superconductivity of
CeOBiS2 without chemical doping is likely triggered by the carriers induced by
the valence fluctuation between Ce3+ and Ce4+.
