In some types of augmented reality (AR), additional information are in different depth planes from that of the real environment. We used rapid serial visual presentation (RSVP) to examine the effect of depth differences in visual attention. Participants observed the RSVP stimuli monocularly. The results of the present experiment were compared with the results from previous study where participants performed the RSVP task binocularly, suggesting that visual attention work harder under monocular-AR than under binocular-AR. We discuss the findings in the context of the safety and visibility of AR where information are presented in different depth.