An element-accommodating package, comprising: a metallic substrate comprising an upper face that comprises a mounting region on which an element such as a semiconductor device, a light-emitting diode, a piezoelectric element, a quartz resonator, a laser diode, or a photo diode can be mounted; a frame disposed on the metallic substrate, comprising: four sides surrounding the mounting region in a plan view of the element-accommodating package; a first through hole (Ha) disposed at one side among the four sides; and a second through hole (Hb) disposed at the one side adjacent to the first through hole (Ha); a first coaxial connector in the first through hole (Ha), comprising a first center core; a second coaxial connector in the second through hole, comprising a second center core; and a circuit board disposed within a region surrounded by the frame and comprising a part being in contact with the one side of the frame in the plan view, wherein an upper face of the circuit board comprises: a first signal line connected to the first center core of the first coaxial connector; a second signal line connected to the second center core of the second coaxial connector; and first ground conductor layers comprising a first ground conductor layer between and spaced from the first signal line and the second signal line, the first ground conductor layers being arranged such that the first and second signal lines are each located between adjacent first ground conductor layers, a lower face of the circuit board comprises: a second ground conductor layer overlapping with the first signal line and the second signal line in the plan view of the element-accommodating package, a groove is disposed between the one side of the frame and a side surface of the circuit board and between the first signal line and the second signal line, the circuit board has a rectangular shape having four corners in the plan view, and comprises a cutout at each of the two corners opposed to the one side of the frame, and the first ground conductor layers are formed at fixed intervals from the groove and the bottom portions of the cutouts in the plan view. a metallic substrate comprising an upper face that comprises a mounting region on which an element such as a semiconductor device, a light-emitting diode, a piezoelectric element, a quartz resonator, a laser diode, or a photo diode can be mounted; a frame disposed on the metallic substrate, comprising: four sides surrounding the mounting region in a plan view of the element-accommodating package; a first through hole (Ha) disposed at one side among the four sides; and a second through hole (Hb) disposed at the one side adjacent to the first through hole (Ha); four sides surrounding the mounting region in a plan view of the element-accommodating package; a first through hole (Ha) disposed at one side among the four sides; and a second through hole (Hb) disposed at the one side adjacent to the first through hole (Ha); a first coaxial connector in the first through hole (Ha), comprising a first center core; a second coaxial connector in the second through hole, comprising a second center core; and a circuit board disposed within a region surrounded by the frame and comprising a part being in contact with the one side of the frame in the plan view, wherein an upper face of the circuit board comprises: a first signal line connected to the first center core of the first coaxial connector; a second signal line connected to the second center core of the second coaxial connector; and first ground conductor layers comprising a first ground conductor layer between and spaced from the first signal line and the second signal line, the first ground conductor layers being arranged such that the first and second signal lines are each located between adjacent first ground conductor layers, a first signal line connected to the first center core of the first coaxial connector; a second signal line connected to the second center core of the second coaxial connector; and first ground conductor layers comprising a first ground conductor layer between and spaced from the first signal line and the second signal line, the first ground conductor layers being arranged such that the first and second signal lines are each located between adjacent first ground conductor layers, a lower face of the circuit board comprises: a second ground conductor layer overlapping with the first signal line and the second signal line in the plan view of the element-accommodating package, a groove is disposed between the one side of the frame and a side surface of the circuit board and between the first signal line and the second signal line, the circuit board has a rectangular shape having four corners in the plan view, and comprises a cutout at each of the two corners opposed to the one side of the frame, and the first ground conductor layers are formed at fixed intervals from the groove and the bottom portions of the cutouts in the plan view.