A circuit board comprising: a ceramic substrate with a thickness of 0.5 mm or less including a first surface and a second surface and; a first metal part including a first metal plate joined to the first surface and a protrusion projecting from the first metal plate, the protrusion being a part of the first metal plate, a part of a lead frame, or a part of a lead pin; and a second metal part including a second metal plate joined to the second surface, wherein a thickness of at least one plate selected from the group consisting of the first and second metal plates (3, 4) is 0.6 mm or more, a ratio of the total volume of the second metal plate to the total volume of the first metal plate is larger than 1.0, when the ceramic substrate is equally divided into first to third sections along a longer side direction thereof, the first metal part has a first region overlapping the first section, a second region overlapping the second section, and a third region overlapping the third section, the second metal part has a fourth region overlapping the first section, a fifth region overlapping the second section, and a sixth region overlapping the third section, and a volume V 1 of the first region, a volume V 2 of the second region, a volume V 3 of the third region, a volume V 4 of the fourth region, a volume V 5 of the fifth region, and a volume V 6 of the sixth region are numbers satisfying formula V 4 /V 1 + V 6 /V 3 ≤ 2 V 5 /V 2, 0.5 ≤ V 4 /V 1 ≤ 2, 0.5 ≤ V 5 /V 2 ≤ 2, and 0.5 ≤ V 6 /V 3 ≤ 2, when the protrusion comprises an external protrusion that includes a protruding portion that protrudes beyond a longer side of the ceramic substrate, the volume of the protruding portion that protrudes beyond the longer side of the ceramic substrate is excluded from the volumes V 1 and V 3, and when the protrusion does not comprise the external protrusion, the volume of the protrusion is included in the volumes V 1 and V 3. a ceramic substrate with a thickness of 0.5 mm or less including a first surface and a second surface and; a first metal part including a first metal plate joined to the first surface and a protrusion projecting from the first metal plate, the protrusion being a part of the first metal plate, a part of a lead frame, or a part of a lead pin; and a second metal part including a second metal plate joined to the second surface, wherein a thickness of at least one plate selected from the group consisting of the first and second metal plates (3, 4) is 0.6 mm or more, a ratio of the total volume of the second metal plate to the total volume of the first metal plate is larger than 1.0, when the ceramic substrate is equally divided into first to third sections along a longer side direction thereof, the first metal part has a first region overlapping the first section, a second region overlapping the second section, and a third region overlapping the third section, the second metal part has a fourth region overlapping the first section, a fifth region overlapping the second section, and a sixth region overlapping the third section, and a volume V 1 of the first region, a volume V 2 of the second region, a volume V 3 of the third region, a volume V 4 of the fourth region, a volume V 5 of the fifth region, and a volume V 6 of the sixth region are numbers satisfying formula V 4 /V 1 + V 6 /V 3 ≤ 2 V 5 /V 2, 0.5 ≤ V 4 /V 1 ≤ 2, 0.5 ≤ V 5 /V 2 ≤ 2, and 0.5 ≤ V 6 /V 3 ≤ 2, when the protrusion comprises an external protrusion that includes a protruding portion that protrudes beyond a longer side of the ceramic substrate, the volume of the protruding portion that protrudes beyond the longer side of the ceramic substrate is excluded from the volumes V 1 and V 3, and when the protrusion does not comprise the external protrusion, the volume of the protrusion is included in the volumes V 1 and V 3.