Retention forestry is a harvesting system that intentionally retains important forest structures at harvest time. We examined the effect of dispersed retention on the productivity and cost of harvesting operations in a large-scale field experiment in conifer plantations in Hokkaido, Japan. For dispersed retention, we retained broad-leaved trees in three levels (10, 50, or 100 trees/ha). We used daily operator reports and investigated time consumption (h/m3) of felling and pre-hauling operations and the total cost to roadside. Compared with clearcutting, mean felling time was 7% and 17% longer, and mean pre-hauling time was 20% and 19% longer in 50 and 100 retained trees/ha, respectively. The other operations were not affected by tree retention because they were conducted at pre-existed strip roads or landings where no trees were retained. Increased time consumption caused an increased cost of felling and pre-hauling. Compared with clearcutting, the combined cost of felling and pre-hauling was 14% and 18% higher in 50 and 100 retained trees/ha, respectively. However, the total cost to roadside increased only by 3% compared with clearcutting because these two operations in total occupied 19% of the overall cost, and the cost of the other operations was not affected by tree retention. This suggests that the impact of tree retention on the total cost was small and that the harvesting system used in the present study can reduce cost increment due to tree retention unless trees are not retained on strip roads.