  We report the sulfur isotope effect on transition temperature in a BiS2-based
superconductor Bi4O4S3. Polycrystalline samples of Bi4O4S3 were prepared using
32S and 34S isotope chemicals. From magnetization analyses, the isotope
exponent (aS) was estimated as -0.1 < aS < 0.1. Although the Tc estimated from
electrical resistivity was scattered as compared to those estimated from the
magnetization, we observed no clear correlation between Tc and the isotope
mass. The present results suggest that unconventional paring states are
essential in Bi4O4S3.
