To provide insulating material with treeing-resistant discharge characteristics improved by controlling the development direction of treeing discharge and a method of manufacturing the insulating material.

The development of treeing discharge is controlled by orientating the insulating material. The treeing-resistant insulating material has a molecular chain or crystal orientation and is used in a manner that the orientation direction is perpendicular to the electric field direction. The degree of orientation is preferably 0.1 or more. Such treeing-resistant insulating material can be manufactured by performing crystallization under pressure after orientation processing. The obtained treeing-resistance insulating material controls the tree development toward the electric field direction, thereby preventing insulation destruction accompanying development of the tree even when high voltage is repeatedly applied.