The performance of a whole system is improved by synchronizing communication and computations between stacked computing LSIs. Each of stacked an external communication LSI and a computing LSI has a PLL which multiplies a crystal oscillator clock signal, a clock pulse generator which distributes the clock signal, and flip-flop circuits. The computing LSI has a DLL circuit composed of a clock phase comparator, a delay controller, and a delay chain. In order to synchronize the communication and computations of the external communication LSI and the computing LSI, a synchronization reference clock signal is transmitted from the external communication LSI to the computing LSI via a through-electrode. An internal clock signal of the computing LSI is synchronized with the synchronization reference clock signal from the external communication LSI by the DLL circuit.