To demonstrate influence of water content of host tees on attacking behavior of Platypus quercivorus and on symbiotic fungi (yeasts and Raffaelea quercivora), the number of beetles landing on bait logs with their water content artificially changed and the number of entry holes bored by the beetles into the bait logs were counted, and the ratio of galleries with eggs or larvae and the ratio of wood pieces with the symbiotic fungi were investigated. A multiple regression analysis of the number of beetles captured and the number of entry holes bored by the beetles was performed with the length and water content of bait logs as explanatory variables. The result suggest that males land on bait logs by judging the length of bait logs or something related to their length, and that males bore into bait logs by judging both the length of the bait logs and water content of the bait logs at the time of breeding or by judging something related to both these factors. At a part with the low water content at the time of breeding, the number of entry holes per m², the ratio of galleries with eggs or larvae, and the ratio of wood pieces containing symbiotic fungi were low, suggesting that P. quercivorus fails to reproduce in a part with low water content, because symbiotic fungi fail to propagate in this part. From the above, it is hypothesized that male P. quercivorus chooses host trees with large breeding capacity by a certain method, resulting in them tending to choose host trees where the symbiotic fungi grows easily because the water content is maintained for a long time.