A transformer comprising: a magnetic core including two base-plates (UCb; DCb) facing each other and four legs (UC1 to UC4, DC1 to DC4) provided between the two base-plates (UCb; DCb) to couple the two base-plates (UCb; DCb) together, the four legs (UC1 to UC4, DC1 to DC4) being arranged along a pair of diagonal lines intersecting each other in a plane along facing surfaces of the two base-plates (UCb; DCb); a primary winding (41A to 41D); and one or more metal plates (421; 422) each having four through-holes (421A to 421D; 422A to 422D) through which the four legs (UC1 to UC4, DC1 to DC4) pass, respectively, and each constituting a secondary winding (42A to 42D) which is wound around the four legs (UC1 to UC4, DC1 to DC4), wherein the primary and secondary windings (41A to 41D; 42A to 42D) are wound around so that closed magnetic paths are formed inside the magnetic core from the four legs (UC1 to UC4, DC1 to DC4) to the two base-plates (UCb; DCb) due to currents which flow through the primary or the secondary winding (41A to 41D; 42A to 42D), and so that a couple of magnetic fluxes each generated inside each of a couple of legs (UC1, DC1; UC3, DC3) arranged along one of the two diagonal lines are both directed in a first direction, while so that another couple of magnetic fluxes each generated inside each of another couple of legs (UC2, DC2; UC4, DC4) arranged along another diagonal line are both directed in a second direction which is opposite to the first direction, characterized in that a printed coil has four through-holes (410A to 410D) through which the four legs (UC1 to UC4, DC1 to DC4) pass, respectively, and constitutes the primary winding (41A to 41D) which is wound around the legs (UC1 to UC4, DC1 to DC4); and that the four legs (UC1 to UC4, DC1 to DC4) are disposed, respectively, at four corners of a square plane of the base-plates (UCb; DCb). a magnetic core including two base-plates (UCb; DCb) facing each other and four legs (UC1 to UC4, DC1 to DC4) provided between the two base-plates (UCb; DCb) to couple the two base-plates (UCb; DCb) together, the four legs (UC1 to UC4, DC1 to DC4) being arranged along a pair of diagonal lines intersecting each other in a plane along facing surfaces of the two base-plates (UCb; DCb); a primary winding (41A to 41D); and one or more metal plates (421; 422) each having four through-holes (421A to 421D; 422A to 422D) through which the four legs (UC1 to UC4, DC1 to DC4) pass, respectively, and each constituting a secondary winding (42A to 42D) which is wound around the four legs (UC1 to UC4, DC1 to DC4), wherein the primary and secondary windings (41A to 41D; 42A to 42D) are wound around so that closed magnetic paths are formed inside the magnetic core from the four legs (UC1 to UC4, DC1 to DC4) to the two base-plates (UCb; DCb) due to currents which flow through the primary or the secondary winding (41A to 41D; 42A to 42D), and so that a couple of magnetic fluxes each generated inside each of a couple of legs (UC1, DC1; UC3, DC3) arranged along one of the two diagonal lines are both directed in a first direction, while so that another couple of magnetic fluxes each generated inside each of another couple of legs (UC2, DC2; UC4, DC4) arranged along another diagonal line are both directed in a second direction which is opposite to the first direction, characterized in that a printed coil has four through-holes (410A to 410D) through which the four legs (UC1 to UC4, DC1 to DC4) pass, respectively, and constitutes the primary winding (41A to 41D) which is wound around the legs (UC1 to UC4, DC1 to DC4); and that the four legs (UC1 to UC4, DC1 to DC4) are disposed, respectively, at four corners of a square plane of the base-plates (UCb; DCb).