To solve the problem that the efficiency of a transformer is reduced when magnetic flux leaks from above and below a bobbin, and upper and lower surfaces of the bobbin come closer to a metal reflecting plate.

The bobbin has a cylindrical winding shaft portion and a winding groove formed between collars provided to the winding shaft portion, wherein a core engagement portion is provided between a first winding portion wound with a first coil and a second winding portion wound with a second coil. A core comprises a rod-shaped first core portion inserted into the cylindrical winding shaft portion of the bobbin, a center core portion extending on the upper surface of the bobbin in parallel to the winding shaft portion, both leg portions which are coupled together by the center core portion and extended so as to be orthogonal from the core portion to both end surfaces of the winding shaft portion of the bobbin, and a second core having a projection portion which is coupled by the center core portion and extended from the center core portion so as to be orthogonal to be engaged with the core engagement portion of the bobbin.