1. A ceramic circuit substrate comprising:a ceramic substrate;
a copper circuit made of a copper-based material bonded, via a first bonding layer, to a surface of the ceramic substrate; and
a copper heat sink made of the copper-based material bonded, via a second bonding layer, to the other surface of the ceramic substrate,
wherein the bonding layers each include a brazing material component comprising at least two or more kinds of metals together with Ag and Cu serving as essential components, at least one active metal, the active metal, in the bonding layers, having a content ranging, relative to a metallic element content in each of the whole bonding layers, from 0.5 mass % to 2.0 mass % inclusive, and the Cu, in each of the bonding layers, having a content ranging from 20 mass % to 40 mass % inclusive,
wherein the bonding layers each have a two-layer structure of:
a brazing material layer comprising the brazing material component but not the active metal, and
an active metal compound layer containing the active metal, the active metal compound layer being formed along a bonded interface between the ceramic substrate and the bonding layer, and
wherein, further, a ratio of a bonding area between the active metal compound layer and the ceramic substrate in a bonding area between each of the bonding layers and the ceramic substrate is 88% or more.