The present invention provides a transformer including a first bobbin (1) around which a primary coil (3) is wound, a second bobbin (2) which is disposed adjacent to the first bobbin (1) and around which a secondary coil (9) is wound, a core disposed across the first and second bobbins and forming a closed magnetic path. The core is divided into a first core (17) positioned on the side where the first bobbin is present and a second core (18) positioned on the side where the second bobbin is present, and an insulating member (14) is interposed in a magnetically coupled portion between the first core (17) and the second core (18), the insulating member (14) including outer circumference sheaths (16a) and (16b) that cover at least one of the first and second cores and a barrier (15) interposed between the opposing surfaces of the first and second cores.