A circuit block assembly is provided that includes circuit blocks that each include a circuit board and a semiconductor element that is disposed on a first main surface of the circuit board. Moreover, each of the circuit blocks includes a metal heat spreader that is connected to the semiconductor element directly or by a thermally conductive member interposed therebetween. A thermally conductive sheet is provided that is thermally connected to the heat spreader. The thermally conductive sheet has a specific electrical resistance higher than a specific electrical resistance of the heat spreader.