A semiconductor device comprising a memory section including a plurality of memory cells having different threshold values, a circuit for reading out existing data from a memory cell into which inputted writing data is written, and a circuit for detecting such a pattern that allows a writing operation to start an erase operation by comparing the writing data with the existing data in the memory cell. The pattern that triggers an erase operation by using a writing operation is regarded as a prohibition operation, and, when the prohibition operation is recognized, a writing operation by a writing command is not started but the operation is ended forcibly. Consequently, such a phenomenon that erase is caused by a program operation can be avoided.