In a multilayer ceramic substrate having a cavity, base-material layers are arranged on a base side with respect to an interface between the base and a wall defining a cavity, and a constraining interlayer is arranged on the wall side. A conductive film is arranged between the base-material layers and the constraining interlayer, the base-material layers and the constraining interlayer sandwiching the interface. The effect of the first conductive film results in an increase in the adhesion of the constraining interlayer to the substrate layers, thus enhancing a shrinkage-inhibiting effect of the constraining interlayer.