Pine wood nematodes  cause  abnormal  cavitation  in Pinus thunbergii tracheids, and the xylem-sap ascent is blocked at cavitated area.  Infected trees wilt for water deficit.  Such cavitation events were not visible from outside of trunks, but can be detected by acoustic emission (AE)  technique.   Prior  to the inoculation of pine wood  nematode,   the  author attached  AE sensors (140kHz) onto the base of pine trunk.  In healthy pines,  AE associated with active transpiration was detected 2 to 6  times/min. during  sunshiny period.  After 1 or 2 weeks after the nematode inoculation on August 4, AE signal abruptly began to continue through nights.  The AE frequency increased to the order of 100 times/min. from 8 to 14 o'clock.  Tracheid cavitation should have developed  drastically  and  widely in the trunks during the period of highly frequent AE detection.   After the high AE period, which was different in individual trees from 1 to 4  weeks, AE suddenly decreased, and disease symptom such as leaf color fading or yellowing became visible.