The continuous planting of Japanese pear leads to a soil sickness syndrome that eventually affects the growth and yield of the plant. In this study, we aimed to elucidate the characteristics of soil sickness syndrome in the Japanese pear and construct countermeasures using the rhizosphere soil assay method that can quantify the risk of soil sickness syndrome by inhibitory chemicals. Water flushing treatment, rainfall treatment, and the incorporation of test soils with different rates of activated carbon were evaluated on the risks of soil sickness. The water flushing treatment under laboratory conditions and exposure of the continuous cropping soil to rainfall in the open field decreased the inhibition rate of the soil. The decrease in soil inhibition rate was presumed to be the result of accumulated growth inhibitory substances in the soil being washed away by water. In addition, activated carbon with the potential to reduce the soil sickness syndrome was selected using the rhizosphere soil assay method. It was clarified that the mixing of the selected activated carbon with the continuous cropping soil reduced the inhibition rate and increased the growth of pear trees increased compared to the untreated soil from the continuous cropping field. The inhibition rate of the soil from the continuous cropping field was reduced to the level of soil with no history of Japanese pear cultivation. In the replanted field, these treatments can promote the growth of trees by reducing the influence of soil sickness syndrome.