And can detect the alteration if one of the speakers of the conversation has been tampered, it provides a time authentication system that can prove that occurred at time within a certain range conversational speech when there is no tampering. In the time authentication authority server 1 to issue all of the speaker in a speaker # 1 terminal 2 and the speaker # 2 terminal 3 difficulty is that other than the time authentication authority server 1 to guess in advance to a unique background sound of conversation Upon request, the time authentication authority server 1 to issue an electronic certificate electronically signed by adding a time stamp to the hash of the background sound and the background sound to the requester of the speaker # 1 terminal 2 and the speaker # 2 terminal 3 . All of the speaker of the conversation, using the speaker # 1 terminal 2 and the speaker # 2 terminal 3, to start recording of conversational speech, including the background sound of all the speakers of the conversation, while playing the background sound and sending a mixture of its own utterance, to save the voice data to end the recording of the conversation voice.