To provide a high efficiency exciter for synchronous motor which can reduce eddy current in the circumferential direction, generated in both cores 13, 15 of a rotary transformer, without sacrifice of power transmission performance.

The stationary side core 13 and the rotary side core 15 of a rotary transformer are constituted, respectively, by combining a plurality of segments 13A, 15A, divided equally in the circumferential direction, annularly. Both cores 13, 15 have such a split structure that three or more division parts do not face each other simultaneously, and the stationary side core 13 is divided into eight while the rotary side core 15 is divided into 7, for example. According to the arrangement, eddy current loss can be reduced because the eddy current in the circumferential direction can be divided. Furthermore, since more than three division parts do not face each other simultaneously, decrease in power transmission efficiency incident to loss of flux transfer can be minimized, and generation of pulsation component can be suppressed.