The pine wilt disease caused by Bursaphelenchus xylophilus (BX), also known as the pine wood nematode (PWN), is the most devastating disease of pine trees. In this work, a high molecular weight B. xylophilus cellulase antigen (BXCa) was purified from total homogenates of nematodes. BXCa was found to be able to hydrolyze carboxymethyl cellulose (CMC) efficiently (155.65 U/mg) and to have an approximate molecular mass of 58.9 kDa. We harvested anti-BXCa antibodies and performed immunocytochemical assays, which revealed the localization of cellulase pools in the esophageal gland cells of the PWN. It was also discovered that cellulase was secreted from the stylet and was used to hydrolyze cellulose to facilitate the PWN entering host cells. These results are consistent with other plant parasitical nematodes. Interestingly, strong fluorescence signals from cellulase staining were observed in tracheid cells in naturally infected pine wood, in addition to ray cells and the resin canal zone. These results strongly suggest that the cellulase released by the PWN is one of the pathogenic substances of pine wilt disease and is responsible for the development of the early symptoms of the disease.