A multilayer substrate is configured by stacking conductive layers and insulation layers. The multilayer substrate includes a core that is one of the conductive layers and is thicker than any of other conductive layers, and a first signal line that is included in the conductive layers and is adjacent to the core so that a first insulation layer that is one of the insulation layers is interposed between the core and the first signal line, the first signal line being used for transmission of an RF signal. The core has a recess portion so as to face the first signal line.