The circuit board includes a ceramic sintered body and a metal wiring layer provided on at least one primary surface thereof with a glass layer interposed therebetween, and when the cross section of the circuit board perpendicular to the primary surface of the ceramic sintered body is viewed, the ratio of the length of an interface between the glass layer and the metal wiring layer to a length of the glass layer in a direction along the primary surface is 1.25 to 1.80.