A semiconductor device to be mounted on a printed wiring board, the semiconductor device comprising: a semiconductor chip including a predetermined semiconductor integrated circuit and an external connection pad configured to connect the semiconductor integrated circuit to an external circuit; an interposer made of silicon or glass; a surface circuit pattern formed on one surface of the interposer, the surface circuit pattern including: chip side pads provided on the surface of the interposer and connected to the external connection pad of the semiconductor chip; junction pads; and interconnecting lines having an end connected to the chip side pads and another end connected to the junction pads, the interconnecting lines extending from the chip side pads toward an outer edge of the interposer; and a post array (30; 300) including a plurality of conducting paths and insulating resin insulating the conductive paths from each other, the post array being arranged such that the conductive paths extend in a direction intersecting with the surface of the interposer, the conducting paths each having an end connected to the junction pad and another end to be connected to the printed wiring board; characterized by a by-pass capacitor for a power supply system of the semiconductor chip being provided on the surface of the interposer on which the surface circuit pattern is formed and in an area of the surface of the interposer overlapping with a mounting area of the semiconductor chip. a semiconductor chip including a predetermined semiconductor integrated circuit and an external connection pad configured to connect the semiconductor integrated circuit to an external circuit; an interposer made of silicon or glass; a surface circuit pattern formed on one surface of the interposer, the surface circuit pattern including: chip side pads provided on the surface of the interposer and connected to the external connection pad of the semiconductor chip; junction pads; and interconnecting lines having an end connected to the chip side pads and another end connected to the junction pads, the interconnecting lines extending from the chip side pads toward an outer edge of the interposer; and chip side pads provided on the surface of the interposer and connected to the external connection pad of the semiconductor chip; junction pads; and interconnecting lines having an end connected to the chip side pads and another end connected to the junction pads, the interconnecting lines extending from the chip side pads toward an outer edge of the interposer; and a post array (30; 300) including a plurality of conducting paths and insulating resin insulating the conductive paths from each other, the post array being arranged such that the conductive paths extend in a direction intersecting with the surface of the interposer, the conducting paths each having an end connected to the junction pad and another end to be connected to the printed wiring board; characterized by a by-pass capacitor for a power supply system of the semiconductor chip being provided on the surface of the interposer on which the surface circuit pattern is formed and in an area of the surface of the interposer overlapping with a mounting area of the semiconductor chip.