PURPOSE: To enable performing a burn-in test for a writing circuit during sufficient time without breaking a memory transistor when screening of the initial failure of the writing circuit is performed.

CONSTITUTION: A writing circuit 2 and a memory transistor 1 are separable, bias voltage required for writing is impressed to the memory transistor 1 from the writing circuit 2 at the time of normal writing operation, and writing is performed. On the other hand, the memory transistor 1 is separated from the writing circuit 2 at the time of the burn-in test, bias voltage is made not to be impressed, and writing is not performed.