Driving is an action that depends strongly on visual information. For displays in the cockpit, a combination of “ease of viewing” to inform the driver of danger early and “annoyance reduction” to avoid drops in the driver’s perception is needed. In this study, we tried to capture “ease of viewing” and “annoyance” in one fixed-quantity indicator. We took up a Camera Monitor System (CMS) as the subject and analyzed the effect that annoyance with the display used in CMSs has on driving behavior. Based on our analysis, we hypothesize that evaluating carelessness in viewing behavior is related evaluating to annoyance. Next, we chose a Detection Response Task (DRT) technique as a method to evaluate driving behavior influenced by this annoyance. We defined the “event detection rate” as a fixed-quantity indicator that takes account of the indirect field of vision provided by the CMS, and applied this indicator to evaluating an electronic rearview mirror and showed that it was effective as a method to measure ease of viewing. At the same time, we showed that the indicator could replace display annoyance after confirming the relationship between passed over events and display annoyance.