An inner diameter of a third stator core is greater than an inner diameter of a second stator core. When a first stator core, the second stator core, and the third stator core are placed in an inner periphery of a coil, a jig is inserted from an inner-periphery opening of a first end side of the third stator core into the third stator core to directly position the first stator core, the second stator core, and the third stator core in a radial direction. Therefore, a side force generated by an axis deviation between the first stator core, the second stator core, and the third stator core can be reduced.