A multilayer dielectric substrate having a cavity formed on a dielectric substrate and having a semiconductor device incorporated within the cavity, a seal ring formed in a rectangular frame shape with two long sides (4a, 4b) and two short sides (4c, 4d), the seal ring being bonded on the multilayer dielectric substrate, the multilayer dielectric substrate comprising a plurality of cavity-resonance suppressing circuits (20A, 20B) that each include: an opening (50A, 50B) formed in a surface-layer ground conductor arranged on a dielectric substrate within the cavity; an impedance transformer (60A, 60B) that is formed in a dielectric substrate, electrically connected via the opening to the cavity, and has a length of an odd multiple of about 1/4 of in-substrate effective wavelength of a first signal wave which is the signal wave with which the semiconductor device is operated; a tip-short-circuited dielectric transmission line (80A, 80B) that is formed in a dielectric substrate and has a length of an odd multiple of about 1/4 of in-substrate effective wavelength of the first signal wave; a coupling aperture (65A, 65B) formed at a part of an inner-layer ground conductor where the impedance transformer is connected to the dielectric transmission line; and a resistor (70A, 70B) formed in the coupling aperture, wherein the multilayer dielectric substrate further comprises: a first cavity-resonance suppressing circuit that suppresses cavity resonance of the first signal wave, a frequency of the first signal wave being fo; and a second cavity-resonance suppressing circuit that suppresses cavity resonance of a second signal wave, a frequency of the second signal wave being fo/2, wherein the first cavity-resonance suppressing circuit is formed at ends of a long side of the seal ring as the long side is divided into three, at a central part of the opposing long side as the long side is divided into three, and at parts of two short sides (4c, 4d) on the long side side as the short sides (4c, 4d) are divided into two, and wherein the second cavity-resonance suppressing circuit is formed at remaining parts along the seal ring. an opening (50A, 50B) formed in a surface-layer ground conductor arranged on a dielectric substrate within the cavity; an impedance transformer (60A, 60B) that is formed in a dielectric substrate, electrically connected via the opening to the cavity, and has a length of an odd multiple of about 1/4 of in-substrate effective wavelength of a first signal wave which is the signal wave with which the semiconductor device is operated; a tip-short-circuited dielectric transmission line (80A, 80B) that is formed in a dielectric substrate and has a length of an odd multiple of about 1/4 of in-substrate effective wavelength of the first signal wave; a coupling aperture (65A, 65B) formed at a part of an inner-layer ground conductor where the impedance transformer is connected to the dielectric transmission line; and a resistor (70A, 70B) formed in the coupling aperture, wherein the multilayer dielectric substrate further comprises: a first cavity-resonance suppressing circuit that suppresses cavity resonance of the first signal wave, a frequency of the first signal wave being fo; and a second cavity-resonance suppressing circuit that suppresses cavity resonance of a second signal wave, a frequency of the second signal wave being fo/2, wherein the first cavity-resonance suppressing circuit is formed at ends of a long side of the seal ring as the long side is divided into three, at a central part of the opposing long side as the long side is divided into three, and at parts of two short sides (4c, 4d) on the long side side as the short sides (4c, 4d) are divided into two, and wherein the second cavity-resonance suppressing circuit is formed at remaining parts along the seal ring. a first cavity-resonance suppressing circuit that suppresses cavity resonance of the first signal wave, a frequency of the first signal wave being fo; and a second cavity-resonance suppressing circuit that suppresses cavity resonance of a second signal wave, a frequency of the second signal wave being fo/2, wherein the first cavity-resonance suppressing circuit is formed at ends of a long side of the seal ring as the long side is divided into three, at a central part of the opposing long side as the long side is divided into three, and at parts of two short sides (4c, 4d) on the long side side as the short sides (4c, 4d) are divided into two, and wherein the second cavity-resonance suppressing circuit is formed at remaining parts along the seal ring.