Beech, Fagus crenata, shift the species distribution northward in the Pleistocene and the Holocene. At present, continuous natural distribution of beech reaches north to the Kuromatsunai Depression, Hokkaido. In addition, dozens of marginal patches and isolated individuals further north beyond this distributional front have been observed discontinuously. Beech grows remarkably well in these small-scattered northern marginal populations, which must have originated from seeds dispersed beyond the northern limit of the continuous beech forest. We have studied a total of 2154 individuals from 33 beech populations using 12 nuclear microsatellite loci to reveal genetic dynamics at the northern front of the range expansion. Genetic diversity parameters such as heterozygosity, allelic richness, and Wright's F_IS, clearly declined toward the northernmost populations. We observed that the effect of genetic drift due to strong isolation was significant in marginal populations at the distributional front. We deduced range expansion process at the northernmost area and discussed the "gene diversity limitation hypothesis", i. e. the low genetic diversity caused the reduction of the expanding speed.