At startup, divided data blocks are received from other authentication devices and are assembled together with the local divided data block to reconstitute first authentication data. After a prescribed time interval, divided data blocks are again received from other authentication devices and are assembled together with the local divided data block to reconstitute second authentication data. The first authentication data and the second authentication data are cross-checked, and the legitimacy of the other authentication devices is decided; in the event that the other authentication devices are legitimate devices, one authentication device among the multiplicity of authentication devices re-divides the second authentication data into a multiplicity of divided data blocks for re-distribution, and a process of reconstituting the authentication data from the divided data, cross-checking the reconstituted data with the first authentication data, and deciding whether the other authentication devices are legitimate devices is repeated at prescribed intervals.