A high-frequency package comprising: a first dielectric substrate having a signal line and a first grounding conductor provided on a back side thereof; a high-frequency element that is connected to the signal line and the first grounding conductor on the back side of the first dielectric substrate by respective first connection conductors; a second dielectric substrate having a signal line and a second grounding conductor provided on a front side thereof that faces the back side of the first dielectric substrate with the high-frequency element mounted thereon and a shielded space therebetween; and a plurality of second connection conductors that connect the first grounding conductor and the second grounding conductor so as to surround the high-frequency element with the shielded space therebetween, wherein: a cavity having sidewalls and a bottom surface is formed under the high-frequency element in the front side of the second dielectric substrate, the cavity having sidewalls and a bottom surface surrounding a cavity space; via-like conductors in the sidewalls and a conductive-layer pattern on the bottom surface of the cavity are connected with the second grounding conductor and surround and shield the cavity space; the shielded space is formed by the first grounding conductor, the second grounding conductor, the plurality of second connection conductors, the via-like conductors and the conductive-layer pattern; and wherein the cavity space is formed under the high-frequency element such that the high-frequency element does not extend into the cavity space. a first dielectric substrate having a signal line and a first grounding conductor provided on a back side thereof; a high-frequency element that is connected to the signal line and the first grounding conductor on the back side of the first dielectric substrate by respective first connection conductors; a second dielectric substrate having a signal line and a second grounding conductor provided on a front side thereof that faces the back side of the first dielectric substrate with the high-frequency element mounted thereon and a shielded space therebetween; and a plurality of second connection conductors that connect the first grounding conductor and the second grounding conductor so as to surround the high-frequency element with the shielded space therebetween, wherein: a cavity having sidewalls and a bottom surface is formed under the high-frequency element in the front side of the second dielectric substrate, the cavity having sidewalls and a bottom surface surrounding a cavity space; via-like conductors in the sidewalls and a conductive-layer pattern on the bottom surface of the cavity are connected with the second grounding conductor and surround and shield the cavity space; the shielded space is formed by the first grounding conductor, the second grounding conductor, the plurality of second connection conductors, the via-like conductors and the conductive-layer pattern; and wherein the cavity space is formed under the high-frequency element such that the high-frequency element does not extend into the cavity space. a cavity having sidewalls and a bottom surface is formed under the high-frequency element in the front side of the second dielectric substrate, the cavity having sidewalls and a bottom surface surrounding a cavity space; via-like conductors in the sidewalls and a conductive-layer pattern on the bottom surface of the cavity are connected with the second grounding conductor and surround and shield the cavity space; the shielded space is formed by the first grounding conductor, the second grounding conductor, the plurality of second connection conductors, the via-like conductors and the conductive-layer pattern; and wherein the cavity space is formed under the high-frequency element such that the high-frequency element does not extend into the cavity space.