A network system where vessel instruments including sensors are connected through a communication network is provided. The sensors generate the same type of navigation data. The system includes a first network for realizing one-to-many communication, a first group instruments connected with each other through the first network, and a second group instruments connected to the first network with a different protocol from the first network. The first group instrument is an input/output gate between the second group instruments and the first network. At least one of the first group instruments has a server function. The first group instruments obtain identification information and types of navigation data of the second group instruments using the first group instruments as the gates, and the server-function instrument obtains identification information and types of navigation data of the instruments. The server function instrument selects one of the sensors generating the same type of navigation data, and outputs a sharing instruction of the data of the selected sensor, through the first network. The first group instrument provides the sharing instruction to the second group instruments using the first group instrument as the gate.