A plurality of units have a transmitter and a receiver. The transmitter puts the unit's own data string into a coding data array using first array information, and calculates an error-correcting code based on the data array in which 0 is put except for the data string. The receiver decodes a data array in which a data string is put into the coding data array based on second array information, using an error-correcting code. The transmitter adds the unit's own data string to a data string, puts the unit's own data string into the coding data array using the first array information on the data string, calculates an error-correcting code for the data array in which 0 is put except for the data string, and determines an error-correcting code of a transmission packet by addition of an error-correcting code of a received packet and the calculated error-correcting code.