<title>Abstract</title>Tuber infection of <italic>Phytophthora infestans</italic> often occurs at harvest. However, it is difficult to accurately estimate the population densities of <italic>P. infestans</italic> in soil, especially Japanese soil. In the present study, <italic>P. infestans</italic> DNA was extracted from soil samples using a modified CTAB-bead method and quantified using real-time PCR to accurately, rapidly and easily estimate the <italic>P. infestans</italic> population densities in upland soils in Japan. <italic>P. infestans</italic> was well quantified in eleven types of soil samples, including nine types of upland soils in Japan, that were artificially inoculated with a zoosporangia suspension. The amounts of <italic>P. infestans</italic> DNA estimated by the real-time PCR were proportional to the inoculum densities. In the non-controlled experimental potato field, <italic>P. infestans</italic> population densities in soil corresponded to the development of symptoms and were correlated with the number of lesions on the potato foliage. These results imply that the proposed real-time PCR assay is suitable for the estimation or monitoring of <italic>P. infestans</italic> population densities in upland soils in Japan. The population densities at the ridge bottoms were larger than those at any other location in commercial potato fields. These results were similar to those of a previous report using a bioassay. Moreover, a correlation between DNA quantity and inoculum potential was observed. In conclusion, the real-time PCR assay developed in this study is suitable for indirect estimation of the inoculum potential of <italic>P. infestans</italic>.