An electrical system apparatus is constructed as follows to easily attain the desired adjustment, repair and change for the version of a stored program on the user side even if a microcomputer containing a rewritable or unrewritable memory is used. That is, the electrical system apparatus includes a one-chip microcomputer containing a memory which is divided into a plurality of blocks and includes one specified block n set in a preset block of the memory for storing a control program for realizing a preset function or specification of the system apparatus, a specified block rewriting program stored in at lest one of the blocks 1 to (n-1) other than the specified block to rewrite the content of the specified block, and a communicating section accessible to the memory and having a serial communicating function, and the rewriting program permits data transmitted from an external device and received by the communicating section to be written into the specified block n.