A communication device for performing communication by employing first and second communication units, includes: a reception unit for receiving a communication packet including a random number generated for every connection with another communication device, a certificate calculated with the random number, and authentication method information indicating whether or not an authentication method at the second communication unit is compatible with the public key system, through the first communication unit; and a method determining unit for determining whether or not an originator of the communication packet accepts public key encryption based on the authentication method information included in the communication packet; wherein in a case of the method determining unit determining that the originator of the communication packet does not accept the public key system, the random number included in the communication packet is replied to the originator as the identification information of the device itself.