Frame transmission source authentication is performed among terminals involved in delivery in a wireless adhoc communication system. A first terminal generates a keyed hashed value by using an authentication header key determined with respect to a second terminal, and gives it to an authentication header of a frame. The second terminal generates a keyed hashed value by using the authentication header key determined with respect to the first terminal, and compares it with the authentication header given to the frame. If the keyed hashed value generated at the second terminal matches the authentication header, it is confirmed that the frame has been transmitted from the first authenticated valid terminal. The first terminal encrypts a payload part by using a unicast encryption key determined with respect to a third terminal. This encrypted payload part can be decrypted only by the third terminal having the unicast encryption key.