A transformer includes a primary coil, a secondary coil, and a core. The core is to be used in combination with the primary coil or the secondary coil, and includes a conductor storage part and a central part. The conductor storage part is a concave recess formed in an annular manner and configured to store a conductor of the coil therein. The central part is a region inward from the conductor when the core is used in combination with the coil. The central part has a recessed shape having an opening that opens toward a direction opposite to a direction of an opening of the concave recess. The central part has an end, at a side opposite to a side of the opening of the central part, which is generally flat so as to be contiguous with an inner opening edge of the conductor storage part.