Abstract: An electronic cash register (ECR) comprising a memory containing addresses identical to those of the mask ROM that permanently stores a variety of trade processing programs and stores data indicating whether any change should be applied to such programs stored in those addresses. Other memories are provided for storing the memory bank data memorizing the addresses of the modified programs among such programs stored in the mask ROM and the modified programs and also storing the modified programs themselves. The electronic cash register embodied by the present invention permits the system to easily and securely write and store a variety of data necessary for changing programs stored in the mask ROM in those memories mentioned above via an input provided for inputting and processing a variety of trading data.