An insulator includes a part to be wound with a coil; a first flange; and a second flange. The first flange is formed at the side of the part closer to a core segment, and includes a coil guide groove that guides the coil to the part. The second flange is formed at the side of the part closer to a distal end of a teeth. The coil guide groove includes a first guide groove and a second guide groove symmetrical with respect to a radial plane. Each of the first and second guide grooves and—extends at an acute angle from an inner surface of the first flange.