PURPOSE: To reduce the number of I/O terminals by connecting a terminal, which is used for input and output commonly, to the connection point between a switch and a control object in the I/O control circuit of a micro processor.

CONSTITUTION: The I/O mode of control circuit 100 is switched by signal S1; and in the output mode, output buffer circuit 101 outputs signal S2 to control light emission diode 33 through load reistance 29. In the input mode, the output of output buffer circuit 101 is inhibited, and the circuit becomes a high-impedance state, and terminal P0 is determined by the switching state of weitch 21, thus determining output signal S3 of input buffer 102.