A multilayer substrate comprising a plurality of stacked heat-press-bonded thermoplastic resin layers (L1 - L4) each having an in-plane conductive pattern provided on one of principal surfaces and an interlayer conductive portion (14, 14a, 14b) provided to penetrate through the thermoplastic resin layer (L1 - L4) in a thickness direction, wherein the plurality of thermoplastic resin layers (L1 - L4) include a first thermoplastic resin layer and a second thermoplastic resin layer a stacking direction of which is inverted with respect to a stacking direction of the first thermoplastic resin layer, the second thermoplastic resin layer being thicker than the first thermoplastic resin layer, wherein one of ends in the thickness direction of the interlayer conductive portion formed at the second thermoplastic resin layer is connected with the interlayer conductive portion (14, 14b) of an adjacent thermoplastic resin layer of the plurality of thermoplastic resin layers, which is adjacent to the second thermoplastic resin layer in the thickness direction such that the in-plane conductive pattern is not interposed therebetween, wherein a) the adjacent thermoplastic resin layer is the first thermoplastic resin layer or a thermoplastic resin layer different from the first thermoplastic resin layer, and, at the interface between the second thermoplastic resin layer and the adjacent thermoplastic resin layer, the interlayer conductive portion formed at the second thermoplastic resin layer has a larger diameter than a diameter of the interlayer conductive portion formed at the adjacent thermoplastic resin layer, or b) the adjacent thermoplastic resin layer is a thermoplastic resin layer different from the first thermoplastic resin layer, and, at the interface between the second thermoplastic resin layer and the adjacent thermoplastic resin layer, the interlayer conductive portion formed at the second thermoplastic resin layer has a smaller diameter than a diameter of the interlayer conductive portion formed at the adjacent thermoplastic resin layer. wherein the plurality of thermoplastic resin layers (L1 - L4) include a first thermoplastic resin layer and a second thermoplastic resin layer a stacking direction of which is inverted with respect to a stacking direction of the first thermoplastic resin layer, the second thermoplastic resin layer being thicker than the first thermoplastic resin layer, wherein one of ends in the thickness direction of the interlayer conductive portion formed at the second thermoplastic resin layer is connected with the interlayer conductive portion (14, 14b) of an adjacent thermoplastic resin layer of the plurality of thermoplastic resin layers, which is adjacent to the second thermoplastic resin layer in the thickness direction such that the in-plane conductive pattern is not interposed therebetween, wherein a) the adjacent thermoplastic resin layer is the first thermoplastic resin layer or a thermoplastic resin layer different from the first thermoplastic resin layer, and, at the interface between the second thermoplastic resin layer and the adjacent thermoplastic resin layer, the interlayer conductive portion formed at the second thermoplastic resin layer has a larger diameter than a diameter of the interlayer conductive portion formed at the adjacent thermoplastic resin layer, or b) the adjacent thermoplastic resin layer is a thermoplastic resin layer different from the first thermoplastic resin layer, and, at the interface between the second thermoplastic resin layer and the adjacent thermoplastic resin layer, the interlayer conductive portion formed at the second thermoplastic resin layer has a smaller diameter than a diameter of the interlayer conductive portion formed at the adjacent thermoplastic resin layer. a) the adjacent thermoplastic resin layer is the first thermoplastic resin layer or a thermoplastic resin layer different from the first thermoplastic resin layer, and, at the interface between the second thermoplastic resin layer and the adjacent thermoplastic resin layer, the interlayer conductive portion formed at the second thermoplastic resin layer has a larger diameter than a diameter of the interlayer conductive portion formed at the adjacent thermoplastic resin layer, or b) the adjacent thermoplastic resin layer is a thermoplastic resin layer different from the first thermoplastic resin layer, and, at the interface between the second thermoplastic resin layer and the adjacent thermoplastic resin layer, the interlayer conductive portion formed at the second thermoplastic resin layer has a smaller diameter than a diameter of the interlayer conductive portion formed at the adjacent thermoplastic resin layer.