PURPOSE: To add and change application without redeveloping a custom LSI circuit by providing the custom LSI circuit with an access program to an aerial address space and a data steal terminal.

CONSTITUTION: A program part in the custom LSI 1 in which programs are fixedly written is provided with an access program to an aerial address space and a bus buffer part in the custom LSI 1 is provided with a data steel terminal. The custom LSI 1 is also provided with a decoding address bus terminal and an I/O controlling terminal. An address decoding part 2 identifies its access by an address signal and a rea/write signal outputted from the custom LSI 1. Ports 7, 8 to be additional function parts can be constituted as additional parts to existing addresses by previously accessing aerial addresses by the program stored in the custom LSI 1 to read out inputs.