  We have investigated the Se isotope effect in layered bismuth chalcogenide
(BiCh2-based) superconductor LaO0.6F0.4Bi(S,Se)2 with 76Se and 80Se. For all
examined samples, the Se concentration, which is linked to the superconducting
properties, is successfully controlled within x = 1.09-1.14 in
LaO0.6F0.4BiS2-xSex. From the magnetization and electrical resistivity
measurements, changes in Tc are not observed for the LaO0.6F0.4Bi(S,Se)2
samples with 76Se and 80Se isotopes. Our results suggest that pairing in the
BiCh2-based superconductors is not mediated by phonons, and unconventional
superconductivity states may emerge in the BiCh2 layers of LaO0.6F0.4Bi(S,Se)2.
