In recent years, a camera monitor system which displays car-mounted camera pictures to in-vehicle displays is being developed. For a future vehicle cockpit, a field of view (FOV) support system with plural displays is assumed, which includes not only a meter and a center display, but also an HUD and the camera monitor system. As a consequence, an area occupied with displays in a driver’s FOV increases. In this situation, coexistence of “easiness in seeing” to tell a driver about dangers early and “annoying reduction” to avoid the driver’s perception drops is needed for in-vehicle displays. In order to meet this requirement, questionnaires (psychological indicator) have been conventionally used. However, here are two difficulties. One is that two items (“easiness in seeing” and “annoying”) are contrary, and the other is that the relation between a result of questionnaires and driver’s security is uncertain. Therefore, determining HMI requirements was difficult when we used the psychological indicator. Accordingly, we considered a behavioral indicator replacing the psychological one to realize “safety driving behavior” which is the essence of the FOV support system. Specifically, we focused on problems of driving behavior (discovery delay and passing over) caused by “annoying” of display, and tried to capture “easiness in seeing” and “annoying” as one fixed-quantity indicator, “response rate of FOV events”. We applied the suggested indicator to evaluations of an electronic rear-view mirror. In the evaluations, we measured the “response rate of FOV events” and observed driver’s behaviors at the same time. As a result, we confirmed that passing over behaviors shown in the indicator reflected “annoying” caused by the display. This means that the fixed-quantity indicator is useful for replacing annoying of display. We consider that the suggested indicator can contribute to determine HMI requirements in the future vehicle cockpit.