PURPOSE: To prevent destruction caused by a junction breakdown, by forming a project ed part in a second drain layer of one conductivity type after a second island-shaped layer of the other conductivity type connected with a first island-shaped layer of the other conductivity type and having the same thickness as that of the first island- shaped layer is formed in a first drain layer of one conductivity type.

CONSTITUTION: A second island-shaped P-type layer 14 is held in the same potential as a first island-shaped P-type layer 13. A voltage across a source electrode wiring 21 and an N+-type drain layer 11 is so applied that the N+-type drain layer 11 has a higher potential than the source electrode wiring 21. Then, a PN-junction between the first island-shaped p-type layer 13 and a N--type drain layer 12 and a PN-junction between the second island-shaped P-type layer 14 and the N--type drain layer 12 are reversely biased and a depletion layer expands downward in accordance with the applied voltage. At this time, when the depletion layer reaches a projection part 11a, breakdown between the second island-shaped P-type layer 14 and the N+-type drain layer 11 is caused, while breakdown between the first island-shaped P-type layer 13 and the N+-type drain layer 11 doesn't take place. In these steps, a source layer is not formed in the second island-shaped P-type layer 14a, and thereby a parasitic NPN transistor is not generated and destruction of the transistor can be prevented.