A vehicle control unit includes interface software memory means storing an interface software program for connecting an application software program with an OS (operating system) in an internal ROM, a CPU (central processing unit) for performing computation for the application software program and the interface software program, a RAM (erasable memory) storing data such as the result of computation, an I/O unit for extending the control unit, and extending means for communicating memory data through a bus or a LAN. A single-chip micro-computer used in vehicle control permits an increase in input/output points or the addition of functionality, in which case an application software program can continue to be used simply by rewriting the interface software program, without further remanufacturing of the core unit.