In an insulator to be inserted into each slot to be attached to adjacent teeth formed at a rotor or a stator of an electric rotating machine, the insulator is composed of insulator segments that are made divisible in a longitudinal direction parallel to a rotational axis of the rotor at a position corresponding to 2 slots and are made divisible in a lateral direction perpendicular to the rotational axis of the rotor, the insulator segments have surfaces to be coupled to each other of a same shape in the lateral direction, and the insulator segments have a creepage distance that is made longer than a distance in the lateral direction, thereby enabling to elongate the creepage distance and to improve work efficiency.