Provided is a data bus system for a micro controller which has an input/output (I/O) unit, a central processing unit (CPU), an internal memory unit, and a peripheral circuitry. The data bus system includes an external access bus used when data is output from the CPU or data is input to the I/O unit or the internal memory unit; an internal access bus used when data is input to the CPU, data is output from the I/O unit or the internal memory unit, or data is input to or output from the peripheral circuitry; and an internal memory test bus used when data is output from the internal memory unit and input to the I/O unit.