PURPOSE: To obtain a write time conforming with a PROM cell, to make to unnecessary current flow since a write mode is not set thereafter, and also to apply no high voltage, by periodically providing the feedback to a writing circuit through a deciding circuit.

CONSTITUTION: A PROM cell 25 is L in its initial state, and in a period of time when an input signal 27 is H, a data of the cell 25 is sent to a buffer 29 through an SA28. A deciding circuit 18 compares 2 values, and if they are same, its output is set to H and L. An NOR circuit 19 becomes H when all input signals are L, turns on a switch 21, and writes in the cell 25. When the signal 27 is H after write is started, the circuit 19 becomes L, and write is stopped. In this period, a state of the cell 25 is inputted to the buffer 29 through the SA28, and is compared by the circuit 18. In this way, write/read codes come periodically. Therefore, a write state of the cell 25 is compared with contents of a write data.