A DC-DC converter using a plurality of transformers capable of decreasing the loss, preventing heat generation of transformers, and improving the heat transfer property of the core, and an integrated type transformer used in this DC-DC converter. A terminal T1 in which a negative electromotive force is generated and a terminal TR2 in which a positive electromotive force is generated, while a switching element Q1 is conducting, are connected at a node N1. An output coil L1 and output terminals TO1 and TO2 are provided on a current route shared by transformers T1 and T2. Diodes D1 and D2 are respectively inserted in a forward direction from a node N3 toward a terminal TR3 and in a forward direction from the node N3 toward a terminal TR4. An operation of a first transformer and an operation of a second transformer are respectively assigned to a flyback operation and a forward operation.