A semiconductor package comprising: a multilayer dielectric substrate in which a recess is formed and a semiconductor device is mounted in the recess; and electromagnetic shielding members (4, 5) that form a cavity on the multilayer dielectric substrate, the multilayer dielectric substrate comprising: a grounding conductor that is an electromagnetic shielding member and is arranged on the multilayer dielectric substrate in the cavity; an opening formed in the grounding conductor; an impedance transformer located in the multilayer dielectric substrate and electrically connected to the cavity through the opening, the impedance transformer including an internal grounding conductor, a ground via and dielectrics between the internal grounding conductor and the ground via; a dielectric transmission line with a short-circuited end, the dielectric transmission line being formed in the multilayer dielectric substrate and including the internal grounding conductor, ground vias (30, 30d), and dielectrics between the internal grounding conductor and the ground vias (30, 30d); and a coupling opening formed in an internal grounding conductor in a connecting section of the impedance transformer and the dielectric transmission line; characterized in that: a distance from the opening to the coupling opening is 1/4 of an in-substrate effective wavelength (λg) of a signal wave in a cavity resonance mode; a distance from the short-circuited end ground via to the coupling opening is 1/4 of an in-substrate effective wavelength (λg) of a signal wave in a cavity resonance mode; and a resistor is formed in the coupling opening. a multilayer dielectric substrate in which a recess is formed and a semiconductor device is mounted in the recess; and electromagnetic shielding members (4, 5) that form a cavity on the multilayer dielectric substrate, the multilayer dielectric substrate comprising: a grounding conductor that is an electromagnetic shielding member and is arranged on the multilayer dielectric substrate in the cavity; an opening formed in the grounding conductor; an impedance transformer located in the multilayer dielectric substrate and electrically connected to the cavity through the opening, the impedance transformer including an internal grounding conductor, a ground via and dielectrics between the internal grounding conductor and the ground via; a dielectric transmission line with a short-circuited end, the dielectric transmission line being formed in the multilayer dielectric substrate and including the internal grounding conductor, ground vias (30, 30d), and dielectrics between the internal grounding conductor and the ground vias (30, 30d); and a coupling opening formed in an internal grounding conductor in a connecting section of the impedance transformer and the dielectric transmission line; characterized in that: a distance from the opening to the coupling opening is 1/4 of an in-substrate effective wavelength (λg) of a signal wave in a cavity resonance mode; a distance from the short-circuited end ground via to the coupling opening is 1/4 of an in-substrate effective wavelength (λg) of a signal wave in a cavity resonance mode; and a resistor is formed in the coupling opening. a grounding conductor that is an electromagnetic shielding member and is arranged on the multilayer dielectric substrate in the cavity; an opening formed in the grounding conductor; an impedance transformer located in the multilayer dielectric substrate and electrically connected to the cavity through the opening, the impedance transformer including an internal grounding conductor, a ground via and dielectrics between the internal grounding conductor and the ground via; a dielectric transmission line with a short-circuited end, the dielectric transmission line being formed in the multilayer dielectric substrate and including the internal grounding conductor, ground vias (30, 30d), and dielectrics between the internal grounding conductor and the ground vias (30, 30d); and a coupling opening formed in an internal grounding conductor in a connecting section of the impedance transformer and the dielectric transmission line; characterized in that: a distance from the opening to the coupling opening is 1/4 of an in-substrate effective wavelength (λg) of a signal wave in a cavity resonance mode; a distance from the short-circuited end ground via to the coupling opening is 1/4 of an in-substrate effective wavelength (λg) of a signal wave in a cavity resonance mode; and a resistor is formed in the coupling opening. a distance from the opening to the coupling opening is 1/4 of an in-substrate effective wavelength (λg) of a signal wave in a cavity resonance mode; a distance from the short-circuited end ground via to the coupling opening is 1/4 of an in-substrate effective wavelength (λg) of a signal wave in a cavity resonance mode; and a resistor is formed in the coupling opening.