A packets sending/receiving apparatus, comprising: authentication and key exchange means; encryption means for producing an encryption sending data; sending condition setting management means for producing sending condition setting information for setting sending condition of the sending packets; packetization means for producing the sending packets using the encryption sending data; receiving condition setting management means for producing receiving condition setting information for setting receiving condition of the receiving packets; packets reception means for receiving the reception packets; and decoding means for decoding the reception data using the decoding key.