In an in-vehicle control device, universal functions, which are universal to all of various corresponding communication modules, are concentrated in a control module. Furthermore, nonuniversal functions, which are not universal to all the communication modules, are concentrated in the respective communication modules. A communication module interface is universal to all the various communication modules. Universal functions, which are universal to all of various vehicular I/O modules, are also concentrated in the control module. Nonuniversal functions, which are not universal to all the vehicular I/O modules, are concentrated in the respective vehicular I/O modules. A vehicular I/O module interface is universal to all the various I/O modules.