An armature includes a multi-phase armature coil and an armature core. The armature core includes a back yoke, a plurality of large-width teeth and a plurality of small-width teeth having a smaller circumferential width than the large-width teeth. The large-width teeth each radially protrude from the back yoke and are spaced from one another in a circumferential direction of the armature core. The large-width teeth have the armature coil concentratedly wound thereon. The small-width teeth each radially protrude from the back yoke and are spaced from one another in the circumferential direction of the armature core. The small-width teeth are arranged alternately with the large-width teeth in the circumferential direction of the armature core. Moreover, 1≤W1/W2≤2, where W1 is a radial width of the back yoke and W2 is the circumferential width of the small-width teeth.