A semiconductor memory device manufactured separately is connected to an interface unit of a semiconductor device. An internal memory formed in the semiconductor device is connected to at least a part of the interface unit. A memory selecting circuit makes the internal memory accessible in a first operation mode, and makes the internal memory inaccessible in a second operation mode. Therefore, for example, putting the semiconductor device into the first operation mode and accessing the internal memory enables the semiconductor device to operate as a predetermined system even when the semiconductor memory device is not connected to the interface unit. The substitution of the internal memory for the semiconductor memory device makes it possible for the semiconductor device to test the interface unit and associated circuits thereof by itself. This consequently allows improvement in the assembly yield of multichip modules.