A system and method is provided for easily detecting a fraudulent user who attempts to obtain authentication using voice reproduced by a reproducer. A personal computer is provided with an audio data obtaining portion for picking up ambient sound around a person as a target of user authentication using voice authentication technology during a period before the person utters, and a fraud determination portion for calculating an intensity level showing intensity of the picked-up ambient sound per predetermined time for each of sections into which the period is divided and for determining that the person is a fraudulent user who attempts to obtain authentication using reproduced voice, when, of two of the calculated intensity levels, the intensity level of the later section is larger than a sum of the intensity level of the earlier section and a predetermined value.