An armature core is formed by a plurality of core pieces connected at connection positions which are the positions included in each of one or more position groups among gcd(Q, P) position groups formed at an equal pitch along the circumferential direction of the armature core, wherein Q is the number of a plurality of slots, P is the number of a plurality of magnets, gcd(Q, P) is the greatest common denominator of Q and P, m is the number of phases, and one position group is a set of m positions separated along the circumferential direction of the armature core by a pitch of an angle of [360/{gcd(Q, P) * m}]degree.