Virtual reality (VR) has been applied to several fields such as entertainment, education, and medicine in recent years. VR is characterized by a high sense of immersion, which can be represented by the attention allocation from the real world to the virtual space. Although a high degree of attention allocation is significant in VR technology, most existing evaluation methods of VR applications are based on subjective questionnaires. Thus, quantitative and objective VR application evaluation methods are needed to realize advanced VR applications. In this study, we adopted a probe stimulus method to evaluate the attention allocation quantitatively and objectively in VR technology. Ten young adult participants underwent an auditory oddball task while they experienced VR content. The amount of attention directed to the VR content could be quantified based on the decrease in the event-related P300 wave response in the case of the oddball task. The participants watched two-dimensional and three-dimensional VR contents on a liquid crystal display and a head-mounted display, respectively, while brain activity was recorded in the form of electroencephalographic signals. A total of 230 probe stimuli at 1800 Hz (standard stimulus), 2000 Hz (target stimulus), and 500 Hz (deviant stimulus) were presented randomly via an earphone for 70 ms at 1000-ms intervals at the fractions of 70, 15, and 15%, respectively. Additionally, the reaction time and false reaction rate during the oddball task were measured as behavioral measures, and a questionnaire was used for subjective evaluation after the task. Based on a comparison of the subjective measure, behavioral measure, and amplitudes of P300 measured with the target stimulus from Pz and deviant stimulus from Cz, we found that attention allocation to the VR content can be quantitatively estimated using the amplitude of P300 for the deviant stimulus. These results suggest that the proposed method involving event-related potentials can be used as an indicator for attention allocation while watching VR content.