A rotor of alternator has core layer units serially located along axial direction. Each unit has a field coil generating magnetic flux and two rotor cores receiving the flux on respective sides of the coil in axial direction. Each core has a first yoke portion located on inner side of the coil, a second yoke portion extending from the first yoke portion toward the outer side and magnetic poles extending from the second yoke portion in the axial direction. The poles of one core and the poles of the other core in each unit extend toward different axial sides and are alternately arranged in circumferential direction on the outer side of the coil. A ratio of the outer circumferential diameter of the first yoke portions to the rotational diameter of the poles is lower than 0.54.