To provide an authentication technique which is useful to a proof of an object existed in a certain place at a certain time.

In an authentication system including a mobile terminal device and a server, the mobile terminal device includes a photographing means, a position information acquisition means, a communication means, and a control means which transmits image information obtained by the photographing means, position information obtained by the position information acquisition means, and identification information of the mobile terminal device through the communication means, and the server includes a communication means, a storage means, a timer, and a control means which stores reception time information of the timer at the time of receiving information transmitted from the mobile terminal device by the communication means of the server and information received by the communication means of the server, into the storage means in association with each other.