A sheet transformer includes a first core having central and lateral legs penetrating a coil on a circuit board, a plate-shaped second core positioned on an end of the first core, the cores being assembled into an integral piece and an air gap being formed between the central leg and the second core, and a fixing member for fixing the cores with respect to the coil. The fixing member has holder portions having claw portions for holding the lateral legs of the first core from a side of the second core respectively, a first spring portion extending from a side of the second core toward the circuit board, for bringing inner plane portions of the legs of the first core into contact with the coil, and a second spring portion for pressing positions of the second core which are opposite to the lateral legs.