A coil insulator of an electrical rotating machine used as a main insulator for insulating a bundle of conductors formed of a plurality of insulated wires to form an armature coil provided in a slot of a rotor core or a stator core of an electrical rotating machine, wherein the coil insulator includes a mica layer including at least one of a peeled-off mica and a composite mica, a woven- or unwoven cloth including at least one of inorganic and organic materials, hexagonal crystal boron nitride particles having a graphitization index of 1.8 or more, and thermosetting polymer organic resin which integrally connects the mica layer and the hexagonal crystal boron nitride particles to each other, and the hexagonal crystal boron nitride particles is oriented in a longitudinal direction of the coil.