The evolution of a species depends on multiple forces, such as demography and natural selection. To understand the trajectory and driving forces of evolution of a target species, it is first necessary to uncover that species' population history, such as past and present population sizes, subdivision and gene flow, by using appropriate genetic markers. Cryptomeria japonica is a long-lived monoecious conifer species that is distributed in Japan. There are two main lines (omote-sugi and ura-sugi), which are distinguished by apparent differences in morphological traits that may have contributed to their local adaptation. The evolution of these morphological traits seems to be related to past climatic changes in East Asia, but no precise estimate is available for the divergence time of these two lines and the subsequent population dynamics in this species. Here, we analyzed the nucleotide variations at 120 nuclear genes in 94 individuals by using amplicon sequencing in combination with high-throughput sequencing technologies. Our analysis indicated that the population on Yakushima Island, the southern distribution limit of C. japonica in Japan, diverged from the other populations 0.85 million years ago (MYA). The divergence time of the other populations on mainland Japan was estimated to be 0.32 MYA suggesting that the divergence of omote-sugi and ura-sugi might have occurred before the last glacial maximum. Although we found modest levels of gene flow between the present populations, the long-term isolation and environmental heterogeneity caused by climatic changes might have contributed to the differentiation of the lines and their local adaptation.