PURPOSE: To realize a bidirectional buffer without requiring a bus terminator by providing a logical gate having a data protecting function to a bus buffer path so as to fix a bus line to a high level or a low level while the bus line is not in use.

CONSTITUTION: An output of a logical gate 21 is connected to an input of a 3- state output buffer 22 and an output of a logical gate 23 is connected to an input of a tri-state output buffer 24. An output of the tri-state output buffer 24 and an input of the logical gate 21 are connected to a connecting terminal 25 of the bus line 1 and an output of the tri-state output buffer 22 and an input of the logical gate 23 are connected to a connecting terminal 26 of the bus line 2. Further, an output/input terminal of a positive feedback logical gate 27 is connected between input and output terminals of the logical gate 21 and output/input terminals of a positive feedback logical gate 28 are connected between input/output terminals of the logical gate 23. The bidirectional bus buffer is obtained with simple constitution like this.