PURPOSE: To execute without a stand-by except the writing to a ROM by guiding out a highly accurate writing command signal to the ROM based on a clock pulse only when the level of the clock pulse is higher than a discriminating level in response to the output of a level discriminator.

CONSTITUTION: When the power is charged, an address signal and a data signal are outputted from a processing circuit 3 to decide whether the clock pulse outputted from a clock pulse generating circuit 7 can be counted or not. When the clock pulse is higher than the predetermined discriminating level, a threshold level, what is called, a control circuit 5 can count the clock pulses. When the clock pulses can be counted, the circuit 5 writes data to a P-ROM 6. In such a way, an electronic cash register 1 can freely process except writing to the P-ROM 6 immediately after the power is charged. Further, the writing can be executed without waiting for a long time.