A semiconductor built-in millimeter-wave band module includes: an insulating substrate made of a mixture containing an inorganic filler and a thermosetting resin; a high thermal conductivity substrate made of a dielectric material having thermal conductivity higher than the insulating substrate and laminated on one surface of the insulating substrate; a plurality of wiring patterns formed on the high thermal conductivity substrate and the insulating substrate; a semiconductor device operating at millimeter-wave band, which is arranged inside of the insulating substrate, is packaged on the high thermal conductivity substrate in a face-up manner, and is connected electrically with the wiring patterns; and a distributed constant circuit element and an active element provided on the semiconductor device. In this module, a void is provided inside of the insulating substrate and in the vicinity of a surface of the distributed constant circuit element and the active element. With this configuration, heat from the semiconductor device operating at a millimeter-wave band can be dissipated effectively and the semiconductor and other circuit components can be packaged with high density.