An alarm device according to the present invention includes a distance measuring sensor for detecting an object in a slanting rear direction of a vehicle and a distance to the object from the vehicle, a direction indicator switch, an alarming object determination part for determining whether the object detected by the distance measuring sensor is an alarming object, and an alarm generation part for generating an alarm to the driver based on the determination result at the alarming object determination part, and so on. The alarming object determination part determines whether the object is an alarming object based on a change with time in the distance between the object detected by the distance measuring sensor and the vehicle, and detection duration of the object by the distance measuring sensor.