An interposer which can prevent an insulating substrate from warping due to heat and a semiconductor device provided with such interposer. The interposer is provided in the semiconductor device with a semiconductor chip. At the time of mounting the semiconductor device on a mounting substrate, the interposer is arranged between the semiconductor chip and the mounting substrate. The interposer is provided with the insulating substrate made of an insulating resin; an island formed on one plane of the insulating substrate with the rear plane of the semiconductor chip bonded through an adhesive; a thermal pad formed on the other plane of the insulating substrate opposite to the one plane, at a substantially opposing position to the island by having the insulating substrate in between; and a thermal via, which is formed by penetrating the insulating substrate from the one plane to the other plane and connects the island and the thermal pad by permitting heat to be transferred.
