A DC/DC converter (1, 1A, 1B) comprising: electronic components group including a first capacitor, a high-voltage-side switching element, a low-voltage-side switching element, an inductor, and a second capacitor and forming a half-bridge circuit; and a substrate including a T-shaped high-voltage region in which a high-voltage wiring pattern having a relatively high voltage is formed, a T-shaped low-voltage region in which a low-voltage wiring pattern having a relatively low voltage is formed, a connection region in which a connection wiring pattern is formed, and a pair of ground regions in each of which a ground pattern is formed, wherein the connection region is positioned as a rectangular region between the high-voltage region and the low-voltage region in a first direction and adjacent to the high-voltage region and the low-voltage region, the pair of ground regions are formed as an rectangular band region along the first direction respectively, the pair of ground regions are positioned adjacent to the high-voltage region, the connection region, and the low-voltage region with a part of the high-voltage region, the connection region, and a part of the low-voltage region interposed between the ground regions in a second direction intersecting the first direction, and interposed between another part of the high-voltage region and another part of the low-voltage region in the first direction, the first capacitor is mounted across any one of the pair of ground regions and the high-voltage region and has one terminal connected with the ground pattern and the other terminal connected with the high-voltage wiring pattern, the high-voltage-side switching element is mounted across the high-voltage region and the connection region and has a drain terminal connected with the high-voltage wiring pattern and a source terminal connected with the connection wiring pattern, the low-voltage-side switching element is mounted across the connection region and the one of the pair of ground regions and has a drain terminal connected with the connection wiring pattern and a source terminal connected with the ground pattern, the inductor is mounted across the connection region and the low-voltage region and has one terminal connected with the connection wiring pattern and the other terminal connected with the low-voltage wiring pattern, and the second capacitor is mounted across the low-voltage region and the one of the pair of ground regions and has one terminal connected with the low-voltage wiring pattern and the other terminal connected with the ground pattern. electronic components group including a first capacitor, a high-voltage-side switching element, a low-voltage-side switching element, an inductor, and a second capacitor and forming a half-bridge circuit; and a substrate including a T-shaped high-voltage region in which a high-voltage wiring pattern having a relatively high voltage is formed, a T-shaped low-voltage region in which a low-voltage wiring pattern having a relatively low voltage is formed, a connection region in which a connection wiring pattern is formed, and a pair of ground regions in each of which a ground pattern is formed, wherein the connection region is positioned as a rectangular region between the high-voltage region and the low-voltage region in a first direction and adjacent to the high-voltage region and the low-voltage region, the pair of ground regions are formed as an rectangular band region along the first direction respectively, the pair of ground regions are positioned adjacent to the high-voltage region, the connection region, and the low-voltage region with a part of the high-voltage region, the connection region, and a part of the low-voltage region interposed between the ground regions in a second direction intersecting the first direction, and interposed between another part of the high-voltage region and another part of the low-voltage region in the first direction, the first capacitor is mounted across any one of the pair of ground regions and the high-voltage region and has one terminal connected with the ground pattern and the other terminal connected with the high-voltage wiring pattern, the high-voltage-side switching element is mounted across the high-voltage region and the connection region and has a drain terminal connected with the high-voltage wiring pattern and a source terminal connected with the connection wiring pattern, the low-voltage-side switching element is mounted across the connection region and the one of the pair of ground regions and has a drain terminal connected with the connection wiring pattern and a source terminal connected with the ground pattern, the inductor is mounted across the connection region and the low-voltage region and has one terminal connected with the connection wiring pattern and the other terminal connected with the low-voltage wiring pattern, and the second capacitor is mounted across the low-voltage region and the one of the pair of ground regions and has one terminal connected with the low-voltage wiring pattern and the other terminal connected with the ground pattern.