A circuit board is provided with a metal wiring layer 12 on at least one principal surface of a ceramic sintered body 11, wherein the above-described metal wiring layer includes a first region 12a which is in contact with the principal surface and which contains a glass component and a second region 12b which is located on the first region 12a and which does not contain a glass component, the thickness of the first region 12a is 35% or more and 70% or less of the thickness of the metal wiring layer 12, and the average grain size in the first region 12a is smaller than the average grain size in the second region 12b.