This study examined whether a participant's level of interest in visual materials could be assessed by event-related potentials to auditory probe stimuli. Twelve young adults performed an auditory target detection task while viewing either interesting or less interesting (neutral) silent video clips. The auditory probe stimuli consisted of target (2000 Hz, p=0.15), standard (1800 Hz, p=0.70), and nontarget deviant (500 Hz, p=0.15) tones. Button press responses to target tones were required. Both target and deviant tones elicited a large P3 wave, the amplitude of which was smaller while participants were viewing interesting video clips than neutral video clips or simple still images (control condition). The amplitude reduction of the P3 to deviant tones was more prominent than that of the P3 to target tones. The difference between the neutral and control conditions was significant only for the deviant P3. The three-tone probe task using perceptually deviant, nontarget stimuli may be a useful objective method to assess how strongly a visual material attracts the viewer's attention.