There is provided a network control device including: network control unit for controlling a communication process in a first network system; a serial interface for controlling a serial communication process between the first network system and a second network system; and initializing unit for updating a communication program of the second network system, wherein the initializing unit has mode switching unit for switching changeable/unchangeable of the communication program of the second network system, and program download unit for transmitting the communication program to the second network system. According to this device, without using a device having a conventional gateway function and changing a conventional device at all, a new different network system is added to a conventional network system such as an in-vehicle network system, so that the network system can be extended.