A transformer includes: a primary winding receiving portion having a primary winding wound around an axis; and a pair of secondary winding receiving portions each having a secondary winding wound around an axis and disposed on the opposite sides of the first primary winding receiving portion with a gap left in the axial direction. The gaps are formed in a size of a value higher than a first predetermined value, and the coupling coefficients between the first primary winding and the two secondary windings are set lower than a second predetermined value by the gaps.