To demonstrate the process of occurrence of mass attack by Platypus quercivorus on living trees and the process of tree death after mass attack, the beetles landing on tree trunks of Quercus crispula and Q. serrata were collected with adhesive paper traps, and the number of entry holes bored by the beetles into the trunks were counted in Kyoto, Japan. Within one week after the first collection of the beetle, both the number of beetles collected and the number of entry holes bored by them suddenly reached a peak simultaneously and then decreased quickly. On the killed tree, it took three weeks from the first collection of the beetle to discoloration of all leaves. To reproduce the mass attack artificially, male beetles were introduced to two trees that had never suffered boring attacks until mid-August. As a result, many beetles landed on these trees and a large number of entry holes were bored. Moreover, almost all leaves of one of these trees had become discolored by 27 days after introduction of the beetles, and the tree had died by the following spring. These results suggest that the first entry holes bored by male beetles triggered the mass attack.