ALERTING SYSTEM AND ALERTING METHOD

An alerting system that, when a risk indicator indicating a risk of a vehicle departing from a lane falls in a predetermined value range, executes a notification at a predetermined conspicuousness level to a driver of the vehicle via an HMI device; decides on a notification parameter set including parameters defining a notification output timing and the conspicuousness level; records presence or absence of a reactive behavior of the driver to the notification executed using the notification parameter set; records reaction information in which a reaction ratio, which is a ratio of the number of the notifications to which the reactive behavior occurs to the number of the notifications executed, is associated with each of the different notification parameter sets; and calculates and decides on, based on the reaction information, an optimal parameter set with which the reaction ratio is estimated to become the highest.