One embodiment is provided with a movement distance measuring means (41) that measures as a movement distance an amount of change per unit of time in a distance to a detected detection target object, an alarm signal output control means (42) that performs control such that an alarm signal (Dout1) is outputted when the measured movement distance is a predetermined determination threshold value or more, and a determination threshold value varying means (43) that varies the determination threshold value to a larger value when a state in which the movement distance is not less than the determination threshold value continues for not less than a first predetermined period and when a state in which the movement distance is less than the determination threshold value and a difference between the determination threshold value and the movement distance is not greater than a predetermined value continues for not less than a second predetermined period.