A client device determines, whether or not transmission of unique data acquired in a host vehicle is necessary on the basis of at least one of a traveling state of the host vehicle and a surrounding environment of the host vehicle, transmits the unique data acquired in the host vehicle in a case where the unique data transmission necessity determining unit determines that the transmission is necessary, determines whether or not transmission of non-unique data acquired in the host vehicle is necessary on the basis of at least one of the driving state of the host vehicle and the surrounding environment of the host vehicle when the host vehicle is determined to be a representative vehicle among multiple vehicles using the unique data acquired in the multiple vehicles, and transmits the non-unique data acquired in the host vehicle when the transmission is determined to be necessary.