Data transfer mode between a plurality of circuit mod modules connected to a data bus is dynamically switched between a time division, space-division multiplexing and so forth for improving data transfer efficiency. For example, the data bus of 4n bit width is used as two data bus of 2n bit width in certain period (transfer mode “b”), as one data bus of 4n bit width in a certain period (transfer mode “a”), and further as four data bus of n bit width in a further certain period (transfer mode “c”). A command of switching of the transfer mode is performed by a transfer control circuit common to respective circuit modules. In response to command of the transfer mode, switching of the bit width of data and switching of the bus connecting condition are performed by the bus adapter circuit.