A controller includes: a bird's eye image synthesizer generating bird's eye image information for displaying a bird's eye image of a work vehicle and a periphery of the work vehicle; a display controller displaying a first image in which a mark representing an object detected by radar devices overlaps the bird's eye image and a second image captured on the same screen of a monitor; an alarm notifying unit notifying, as the alarm, at least one of a first alarm of displaying a region with the mark differently from a region without the mark in the first image and a second alarm of notifying the existence of the object by a sound; and an alarm releasing unit releasing the alarm in accordance with a releasing order based on a position in which the object exists.