In a multilayer board, a transmission line includes layers including a first insulator layer, a first joining material layer in contact with a first surface of the first insulator layer, and a second joining material layer in contact with a second surface of the first insulator layer. A signal conductor of the transmission line is on the first surface of the first insulator layer, a relative permittivity of the second joining material layer is lower than a relative permittivity of the first joining material layer, and an adhesion strength between the first insulator layer and the first joining material layer is higher than an adhesion strength between the first insulator layer and the second joining material layer.