To achieve stable communication while reducing load related to rigging.

First radio devices 11a and 11b transmit engine information on a first cycle by using individual preset communication channels. A second radio device 23a determines the presence or absence of receipt of the engine information on a second cycle shorter than the first cycle. In the presence of information received, the second radio device performs processing of the received engine information, and then changes the communication channel. In the absence of information received, the second radio device promptly changes the communication channel.