A fluoride-ion conductor, gamma-PbSnF4, was synthesized by the mechanical milling. In addition,)beta-PbSnF4 was obtained by aging the gamma-PbSnF4 at 473 K. The electrical conductivity of beta-PbSnF4 is relatively higher than that of gamma-PbSnF4 at 298 K. The crystal structure analysis of gamma- and beta-PbSnF4 was carried out using neutron diffraction data. From the obtained occupancies, fluoride ions were located at the F-gamma(1) normal site (62%) and F-gamma(2) interstitial site (38%) in gamma-PbSnF4 and the F-beta(1) normal site (31%), F-beta(2) normal site (25%), and F-beta(3) interstitial site (44%) in beta-PbSnF4. In particular, the number of fluoride ions at the F interstitial site increased after the gamma-to-beta phase transition: 38% at F-gamma(2) to 44% at F-beta(3). It is most likely that the ratio of fluoride ions to vacancies (or the effective carrier concentration) was optimized in the "-F-beta(1)-F-beta(3)-F-beta(3)-F-beta(1)-" conduction pathways of fluoride ions in beta-PbSnF4.