A control unit of an alarm device suppresses the activation of an alarm for a second another vehicle when a self-vehicle is reversing from a parked state, in a case where after one of a first detection unit, which is configured to detect another vehicle approaching from the back right of the self-vehicle, or the second detection unit, which is configured to detect another vehicle approaching from the back left of the self-vehicle, detects a first another vehicle, another one of the first detection unit or the second detection unit detects the second another vehicle, in a case where a distance difference between a first distance between the first another vehicle and the self-vehicle and a second distance between the second another vehicle and the self-vehicle is less than or equal to a predetermined distance, in a case where a speed difference between a first speed of the first another vehicle and a second speed of the second another vehicle is less than or equal to a predetermined speed, or in a case where a time difference between a first estimated crossing time of the first another vehicle and the self-vehicle and a second estimated crossing time of the second another vehicle and the self-vehicle is less than or equal to a predetermined time.