Pittosporum parvifolium, belonging to the family Pittosporaceae, is a tree species endemic to Chichijima Island, the Ogasawara (Bonin) Islands. It is classified as a critically endangered species because <10 survive. We investigated the plausible causes for the mortality of a P. parvifolium tree that died in 2014, including associated fungi and insects within the wood and surrounding environmental conditions of the dead tree. We detected a saprophytic fungus Marasmiellus sp. and an endemic ant species Camponotus ogasawarensis that only nests in dead wood. These were not considered to be associated with the tree mortality. In contrast, a longicorn species that formed many borings and produced frass inside of the wood was identified to be Curtomerus flavus. This species is an invasive longicorn of the Ogasawara Islands, attacks living tissues of declining trees and sometimes kills such trees. Spindly growth of the surrounding trees of the P. parvifolium was observed during the past 5 years before the death. Therefore, the cause for P. parvifolium mortality was a decline by suppression, followed by an insect’s attack.