Retention forestry is a silvicultural system which retains important forest structures and organisms at the time of harvesting and maintains them for a long time. Retention forestry could contribute to conserving biodiversity, however, its effect on biodiversity has been investigated in limited regions.  We launched a large-scale experiment in native fir-plantation in Hokkaido, Japan. There are 5 treatments (clear-cutting, the three levels of dispersed retention, and group retention) and 2 controls (unharvested fir-plantation and unharvested natural forest). In dispersed retention sites, broadleaved trees were retained apart from each other at three levels (10, 50, and 100 trees/ha), while in group retention sites, a forest patch of fir-plantation (0.36 ha) was retained. We focused on ground beetles and examined the changes in species assemblages before and after the harvest. We found that harvesting decreased the abundance of most of the forest species, while the increased level of dispersed retention trees can mitigate the decrease in these species. It suggests that retained trees can improve the survival and/or re-colonization rate of forest species. We also found that some forest species increased at forest patches in group retention sites, indicating that a retained patch can provide a refuge for these species after harvest.