PURPOSE: To decrease an erasure time of an ROM by adding an erasure state detecting means detecting whether or not the ROM is erased completely and an erasure state display means displaying the result of detection during erasure.

CONSTITUTION: A controller 40 gives an address signal (a) to all IC2 while irradiating an ultraviolet ray so as to erase the ROM2 and reads the content of an address 0000 of the required IC2 by selecting one output among all outputs of the IC2 by an IC selector 32. Then the value is compared with an FF by a comparator 33 and when they are not identical, since the IC2 is not erased completely, the content of the next IC2 is checked. When they are identical, since the IC2 is erased completely, "1" is added to the address and an address signal of the next address is fed. When the content of the memory of the IC2 selected at present becomes FF is this way, the controller 40 gives of display control signal to an LED display device 50, an LED 21 placed at a position corresponding to the IC2 is lighted so as to display the erasure.