PURPOSE: To decrease the number of lines by disabling a level discriminating circuit just for a time longer than the changing delay time of a signal sent from a slave device owing to the stray capacity generated between lines by using a disabling circuit which is provided in response to the level disabling circuit.

CONSTITUTION: A disabling circuit 70 contains a comparator M6 and a buffer 71 which receives the signal from a resistance R15. The buffer 71 has a high input impedance and sets an output terminal 72 at a high and low level when the input of the buffer 71 is set at a high and low level respectively. The terminal 72 charges quickly a capacitor C via a diode D14, etc. connected forward to a resistance R32. The output voltage of the capacitor C is inverted by an inverter 74. Then the inverter 74 sets its output at a low potential and delivers a signal of a high potential when the level of the input terminal is higher and lower than the discriminating level respectively.