A control unit for vehicle is provided where an increase in a number of input/output and upgrading functions can easily be performed and, in addition to this, changing of a program in a control unit can easily be performed even when a single-chip micro-computer is used. A control unit is constructed such as to comprise an interface software memory storing an interface software program for connecting an application software program with an operating system in an internal ROM, a CPU for performing computation of the application software program and the interface software program, a RAM storing data such as the result of the computation, an I/O unit for extending the control unit, and an extending device for communicating memory data through a bus or a LAN. A single-chip micro-computer used in vehicle control can easily cope with an increase in the input/output ports or additional functions, and an application software program can continuously be used only by rewriting an interface software program. Further, re-manufacturing of a core unit is unnecessary. Therefore, development of a control unit including programs becomes easy.