1. An in-vehicle control device for a vehicle, the in-vehicle control device comprising: a control module that is modularized to control operation of the entire in-vehicle control device; a detachably installed communication module that is detachably connected to the control module through a communication module interface and is modularized to communicate with an external communication network; and a detachably installed vehicular I/O module that is detachably connected to the control module through a vehicular I/O module interface and is modularized to communicate data with an in-vehicle LAN of the vehicle, wherein: the control module includes: at least one universal function that is universal to the detachably installed communication module and at least one replaceable communication module, wherein each replaceable communication module is replaceable with the detachably installed communication module and uses a communication method that is different from a communication method of the detachably installed communication module; and at least one universal function that is universal to the detachably installed vehicular I/O module and at least one replaceable vehicular I/O module, wherein each replaceable vehicular I/O module is replaceable with the detachably installed I/O module and meets an in-vehicle LAN standard that is different from an in-vehicle LAN standard met by the detachably installed vehicular I/O module; the detachably installed communication module has at least one nonuniversal function that is not universal to the detachably installed communication module and the at least one replaceable communication module; the communication module interface is universal to the detachably installed communication module and the at least one replaceable communication module; the detachably installed vehicular I/O module has at least one nonuniversal function that is not universal to the detachably installed vehicular I/O module and the at least one replaceable vehicular I/O module; and the vehicular I/O module interface is universal to the detachably installed vehicular I/O module and the at least one replaceable vehicular I/O module.