PURPOSE: To prevent a substrate potential from floating and transistor(TR) characteristics from varying owing to an increase in substrate current by connecting a TR in series with an output TR and providing another TR between its gate and source.

CONSTITUTION: A TR 3 is connected in series to the power source side of output TRs 1 and 2, a TR 4 is arranged between the gate and source, and its gate is controlled with the output of a writing circuit WE. The TRs 1∼3 are off at the time of writing, so an output terminal Dout is in a high impedance state. At this time, if the potential at the output terminal drops below the ground potential owing to an external noise and the value decreases by more than A threshold value, the TR 2 turns on and the potential at the contact A falls to the same potential. Here, a TR 4 is on, so the gate of the TR 3 is at the same potential with a contact A, so the TR 3 stays off and no current flows from a power source even if the potential at the output terminal falls below 0V.