A built-in-electronic-component substrate includes a core substrate, an electronic component mounted on one main surface of the core substrate via a joining member, and a resin layer including the electronic component embedded therein. The electronic component is a multilayer ceramic capacitor including a ceramic multilayer body, and a first outer electrode including an end surface portion and a second outer electrode including an end surface portion provided on end surfaces of the ceramic multilayer body. A first gap is provided between the resin layer and the end surface portion of the first outer electrode and the joining member and a second gap is provided between the resin layer and the end surface portion of the second outer electrode and the joining member.