PURPOSE: To enable a reverse current caused by a parasitic diode formed between the source and the drain of a MOS transistor to be blocked by a simple means.

CONSTITUTION: A MOS transistor element 2 is formed on a semiconductor substrate 1, where a reverse current blocking diode 16 opposite in polarity to a parasitic diode 12 formed between the source and the drain of the transistor element 2 is built in the MOS transistor element 2. The MOS transistor concerned is of an N-channel type or a P-channel type, and if the MOS transistor is of an N-channel type, a P-N junction between an N--type impurity region 13 and a P--type impurity region 14, which is opposite to the P-N junction between a P--type impurity region 4 and the N--type impurity region 13 where the parasitic diode 12 is formed, is formed, whereby a reverse current blocking diode 16 can be built in.