A circuit substrate includes: a substrate body and an electrode. The substrate body has four sides including first and second sides extending in a first direction and opposing each other in a second direction. The substrate body has a top surface on which the electrode is mounted. The electrode has first and second outer points positioned closest to the first and second sides in the second direction respectively. The top surface has: a first area between the first side and a first imaginary line extending in the first direction and passing through the first outer point; a second area between the second side and a second imaginary line extending in the first direction and passing through the second outer point; and a third area interposed between the first area and the second area in the second direction, the substrate body being formed with a first opening in the first area.