Soil structure is heterogeneous with cracks or macropores, where bypass flow may lead to applied chemicals avoiding interaction with soil particles or the contaminated area.We investigated the bioremediation efficiency of oil-contaminated soils by applying suction at the bottom of soil columns during the bioremediation.Unsaturated flow was investigated so as to avoid bypass flow and achieve mutual dispersion of the chemicals in the soil column.The boundary conditions were 0kPa (saturated flow) and-3kPa (unsaturated flow), and were applied to a soil with and without macropores.Unsaturated flow was achieved with-3kPa and an injection rote of 1/10 of saturated hydraulic conductivity.Resultant biological activities of the effluent increased dramatically in the unsaturated flow with macropores conditions.Unsaturated conditions prevented bypass flow and allowed dispersion of the injected nutrients.Differences in biological activity between unsaturated flow conditions with and without macropores were probably caused by oxygen supply.Thus, as long as bypass flow is avoided, macropores can work as an air supply system.Residual oil concentrations were almost the same for saturated and unsaturated flow conditions, showing that the same purification was achieved with 1/10 of the nutrient application.These results suggest that effective use of nutrients or remediation chemicals is possible by avoiding bypass flow and enhancing biological activity using relatively simple and inexpensive techniques.