Layered superconductors have been attracted substantial attention since the discovery of the cuprate and Fe-based high-Tc superconductors due to the diversity in their crystal structures and the fascinating superconducting states. We have synthesized a new BiCh2-based superconductor system, La1-xCexOBiSSe. Zero resistivity and clear diamagnetic susceptibility were obtained for x = 0.2-0.6, implying the emergence of bulk superconductivity. Dome-shaped superconductivity phase diagram with the Tc of 3.1 K for x = 0.3-0.5 was established. We propose here to investigate the emergence of bulk superconductivity via mixed valence of Ce ions in layered bismuth-chalcogenide systems using ZF and TF MUSR measurements on the MUSR spectrometer. The results in this work are expected to enhance the understanding of the mechanisms of superconductivity in the BiCh2-based superconductor family.