The premise of driving support systems is to reduce a driver's load and traffic accidents as a whole. However, we feared that such systems can degenerate a driver's ability by causing an overreliance on driving assistance systems consciously and/or unconsciously. Therefore, we previously produced an assistance system that reduces encounters with accident factors by helping the driver turn their attention back to safe driving methods. This paper examined the timing of appropriate the assistance system. In previous experiments, the driver was preferred timing that find for event immediately after providing information. Therefore, we installed the system in a driving simulator and conducted an experiment of appropriate timing by changing the field of view in the day and night. As a result, the suitable timing has changed. However it is reasonable to provide at a littele earlier timing.