Two-year-old seedlings of Japanese red pine were inoculated with Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle, the nematode causal agent of pine wilt disease, in three experimental plots. The plots were maintained under continuous radiation of 2, 9, and 35 klx (27, 155, and 660 μE∙m−2∙s−1) light intensity at 25 °C. The number of nematodes inoculated was 5000 per seedling. Development of the disease was faster at 2 and 9 klx than at 35 klx light intensities, total mortality rates 63 days after inoculation being 97, 90, and 13%, respectively. No differences were found among the three experimental plots in the number of nematodes extracted from killed seedlings. During the early days after inoculation, the photosynthetic rate of inoculated as well as control seedlings was distinctly accelerated with these increasing light intensities. The photosynthetic ability of the inoculated seedling under the 2-klx regime became abnormal 7 days after inoculation, whereas under the 9- and 35-klx regimes, it became abnormal 12 days later. The first external symptom on the seedlings used for the measurement of photosynthetic rate appeared from 27 to 31 days after inoculation. These results demonstrate that the development of the pine wilt disease is strongly correlated with the photosynthetic rate of pines, and that this photosynthetic ability is reduced considerably before the first symptoms appear.