A transformer comprising: a coil bobbin which includes a pair of core winding portions and a pair of coupling portions for coupling the core winding portions so as to space the core winding portions a predetermined distance from each other and is formed, on its whole outer periphery, with a groove; a winding which is obtained by winding a conductor around the groove of the coil bobbin a predetermined number of times; and a pair of wound cores each of which is obtained by winding an electromagnetic steel plate around each of the core winding portions of the coil bobbin a predetermined number of times; wherein an outer peripheral surface of each of opposite side walls of the groove at the core winding portions is curved so as to have an arcuate cross-sectional shape.