A user of a mobile phone photographs himself/herself, and sends a password to an authentication server. After the authentication server authenticates the user, the mobile phone stores the photographed image of the user in a memory accessible only by authentication software. When the authentication software detects a billing apparatus at the time the user makes a purchase, the image is read from the memory and displayed while an indicator is on. A store clerk can confirm whether the person carrying the mobile phone is the legitimate user by judging whether the face image matches the person and whether the indicator is on at the same time.