A semiconductor device comprising: a base substance; a first semiconductor chip and a second semiconductor chip installed on the base substance; a first coil provided on the base substance, and electrically connected to the first semiconductor chip; a second coil provided on the first coil, electromagnetically connected to the first coil, and electrically connected to the second semiconductor chip; and a transformer assembly (18, 18A, 18B) installed on the base substance, and made up by: a first core section (181, 181A, 181B) extending from the center of the second coil to the center of the first coil; a first side shield section (182, 182A, 182B) extending along a part of the side surface of the second coil and a part of a side surface of the first coil; and a first upper shield section (183, 183 A, 183B) provided on the first coil and the second coil, the first core section (181, 181A, 181B), the first side shield section (182, 182A, 182B) and the first upper shield section (183, 183A, 183B) being made of ferromagnetic materials; characterized in that, the base substance is made of a ferromagnetic material; and the semiconductor device further comprises: a sealing body covering a part of the base substance, the first semiconductor chip, the second semiconductor chip, the first coil, the second coil, and the transformer assembly (18, 18A, 18B); a third coil provided on the base substance at a position different from the position of the first coil and the second coil, and electrically connected to the first semiconductor chip; and a fourth coil provided on the third coil, electromagnetically connected to the third coil, and electrically connected to the second semiconductor chip, wherein the transformer assembly (18, 18A, 18B) further includes: a second core section (184, 184A, 184B) extending from the center of the fourth coil to the center of the third coil; a second side shield section (185, 185A, 185B) provided along a part of a side surface of the fourth coil and a part of a side surface of the third coil; a second upper shield section (186, 186A, 186B) provided on the third coil and the fourth coil; and a transformer shield section (187, 187A, 187B) provided between the first coil and the second coil, and between the third coil and the fourth coil. a base substance; a first semiconductor chip and a second semiconductor chip installed on the base substance; a first coil provided on the base substance, and electrically connected to the first semiconductor chip; a second coil provided on the first coil, electromagnetically connected to the first coil, and electrically connected to the second semiconductor chip; and a transformer assembly (18, 18A, 18B) installed on the base substance, and made up by: a first core section (181, 181A, 181B) extending from the center of the second coil to the center of the first coil; a first side shield section (182, 182A, 182B) extending along a part of the side surface of the second coil and a part of a side surface of the first coil; and a first upper shield section (183, 183 A, 183B) provided on the first coil and the second coil, the first core section (181, 181A, 181B), the first side shield section (182, 182A, 182B) and the first upper shield section (183, 183A, 183B) being made of ferromagnetic materials; characterized in that, the base substance is made of a ferromagnetic material; and the semiconductor device further comprises: a sealing body covering a part of the base substance, the first semiconductor chip, the second semiconductor chip, the first coil, the second coil, and the transformer assembly (18, 18A, 18B); a third coil provided on the base substance at a position different from the position of the first coil and the second coil, and electrically connected to the first semiconductor chip; and a fourth coil provided on the third coil, electromagnetically connected to the third coil, and electrically connected to the second semiconductor chip, wherein the transformer assembly (18, 18A, 18B) further includes: a second core section (184, 184A, 184B) extending from the center of the fourth coil to the center of the third coil; a second side shield section (185, 185A, 185B) provided along a part of a side surface of the fourth coil and a part of a side surface of the third coil; a second upper shield section (186, 186A, 186B) provided on the third coil and the fourth coil; and a transformer shield section (187, 187A, 187B) provided between the first coil and the second coil, and between the third coil and the fourth coil. a sealing body covering a part of the base substance, the first semiconductor chip, the second semiconductor chip, the first coil, the second coil, and the transformer assembly (18, 18A, 18B); a third coil provided on the base substance at a position different from the position of the first coil and the second coil, and electrically connected to the first semiconductor chip; and a fourth coil provided on the third coil, electromagnetically connected to the third coil, and electrically connected to the second semiconductor chip, wherein the transformer assembly (18, 18A, 18B) further includes: a second core section (184, 184A, 184B) extending from the center of the fourth coil to the center of the third coil; a second side shield section (185, 185A, 185B) provided along a part of a side surface of the fourth coil and a part of a side surface of the third coil; a second upper shield section (186, 186A, 186B) provided on the third coil and the fourth coil; and a transformer shield section (187, 187A, 187B) provided between the first coil and the second coil, and between the third coil and the fourth coil.