A mounting region is provided at an approximately center of one surface of an insulating layer. A conductive trace is formed so as to outwardly extend from inside of the mounting region. A cover insulating layer is formed in a periphery of the mounting region so as to cover the conductive trace. A terminal of the conductive trace is arranged in the mounting region, and a bump of an electronic component is bonded to the terminal. A metal layer made of copper, for example, is provided on the other surface of the insulating layer. A pair of slits is formed in the metal layer such that a region being opposite to the electronic component is sandwiched therebetween. Each slit is formed so as not to divide the metal layer into a plurality of regions.