A composite substrate includes a resin substrate and a low-temperature fired ceramic bonded to the resin substrate in a thickness direction. The low-temperature fired ceramic is a substantially rectangular cuboid having a first main face, a second main face opposing the first main face, and side faces connecting the first main face and the second main face. The first main face is directly bonded to the resin substrate. Corners between the first main face and the side faces are rounded with a radius R1. Corners between adjacent side faces are rounded with a radius R2. The radius R2 is greater than the radius R1.