A semiconductor device has an SOI (Silicon On Insulator) structure and comprising a P-channel FET and an N-channel FET which are formed on an insulating film. The semiconductor device includes: at least two of first, second, third and fourth PN-junction elements. The first PN-junction element is formed from a P-type semiconductor layer and an N-type semiconductor layer having the same impurity concentrations as those of source/drain regions of the P-channel FET and the N-channel FET, respectively. The second PN-junction element is formed from a P-type semiconductor layer and an N-type semiconductor layer having the same impurity concentrations as those of the source/drain region and a channel region in the P-channel FET, respectively. The third PN-junction element is formed from a P-type semiconductor layer and an N-type semiconductor layer having the same impurity concentrations as those of a channel region and the source/drain region in the N-channel FET, respectively. The fourth PN-junction element is formed from a P-type semiconductor layer and an N-type semiconductor layer having the same impurity concentrations as those of the channel regions of the N-channel FET and the P-channel FET, respectively. At least two PN-junction elements are connected in series in a forward bias between two different terminals to form an electrostatic protection device.