A multilayer dielectric substrate includes a first cavity-resonance suppressing circuit that suppresses cavity resonance of a first signal wave and a second cavity-resonance suppressing circuit that suppresses cavity resonance of a second signal wave, a frequency thereof being different from that of the first signal wave. These cavity-resonance suppressing circuits respectively include openings formed in a surface-layer ground conductor, an impedance transformer with a length of an odd multiple of about ¼ of in-substrate effective wavelength of a signal wave, a tip-short-circuited dielectric transmission line with a length of an odd multiple of about ¼, of in-substrate effective wavelength of a signal wave, a coupling aperture formed in an inner-layer ground conductor, and a resistor formed in the coupling aperture. The multilayer dielectric substrate that suppresses cavity resonance of signal waves of a plurality of frequencies.