To provide a method for effectively healing damaged tree low in effects on ecosystem by preventing decay of trunk caused by invasion of a different kind of pest insect or pathogenic bacteria to the damaged part.

A method for controlling insect pest of a tree by injecting a systemic translocating insecticide to a trunk, perforates a hole of 20-300 mm depth on the trunk of the damaged tree by a drill having 2-30 mm diameter after detecting aphids and insects imparting damage to the tree, and then injecting 0.02-1.50 g of the systemic translocating insecticide as the active ingredient per 10 mm diameter of the trunk at a chest height, closing the hole with silicone, cork or the like and the liquid medicine translocates to the trunk or leaves with tree's water supply to control parasitic aphids and insects at the place.