The natural farming system consists of the following farming practices: 1) no/minimum tillage, 2) no weed elimination, 3) no agrochemicals, and 4) no chemical fertilization. This system can be sustainable, because soil disturbance is minimized thus soil structure is favorable for plant growth and minimize soil erosion, and also nutrient cycling in the field is highly closed. It is recommended to convert upland cropland from conventional to natural farming in order to protect soil and soil biodiversity. However, conversion into this system from conventional farming often takes 7 to 10 years. We examined temporal changes (0 to 17 years after conversion) in soil and soil biodiversity in a natural farm in central Japan. Soil carbon accumulation rate was estimated to be ca. 41 g/yr, and abundances and biomass of macrofauna and mircroarthropods were increased after 5 to 10 years. These results suggest that soil changes happened during 5 to 10 year after conversation and the conservation measures of cropland soil encourage soil biodiversity and hence soil quality.