An insulator of an armature includes an upper resin member and a lower resin member. Each of the upper and lower resin members includes decreased thickness portions. Axial positions of the decreased thickness portions of the upper and lower resin members are arranged to overlap at least partially with each other. Each of the upper and lower resin members further includes a rib arranged to project from a corresponding one of the decreased thickness portions. The rib improves the strength of the corresponding decreased thickness portion and reduces the likelihood that any of the decreased thickness portions of the upper and lower resin members will be damaged when the two members are fitted to each other.