To provide a structure of a core of a motor that brings about good yield of a core material and that improves workability.

In a structure of the core of a motor, in which the core 1 of a cylindrical shape is divided equally into 'm' pieces and (n-1) pieces of calking protrusions 2 are provided equally on each segment, (m×n) is made relatively prime with respect to the number of field poles 'p' of the motor. Also, in a structure of the core of a motor, in which the divided portions 3 of the core 1 of the cylindrical shape and the positions of the calking protrusions 2 are arranged with an equal interval, (360/θ) is made mutually prime to the number of field poles 'p', when the angle of the equal interval is 'θ'.