In a semiconductor circuit device having a substrate, a first region of a first conductivity type formed in the substrate, a second region of a second conductivity type contacted to the first region and a MISFET formed in the second region, there is a problem of that a parasitic npn bipolar transistor constituted by the first region, the second region and a source or drain of the MISFET activates. In this invention, switching circuitry is provided to make the second region floating or to connect the second region with the source or drain of the MISFET when a negative first input voltage is input to a source or a drain of the first MISFET. By virtue of the switching circuitry, no base current of the parasitic bipolar transistor occurs, thereby preventing operation of the parasitic transistor. In particular, when the switching circuitry operates to cause the second region to float, the base of the parasitic bipolar transistor will float, and the bipolar transistor cannot operate. Alternatively, when the switching circuitry connects the second region with the source or drain, the base-emitter junction of the parasitic bipolar transistor will be shorted, which also prevents its operation.