PROBLEM TO BE SOLVED: To provide a reflection-type ultrasonic sensor suitable for being applied to a living body detection device.SOLUTION: A living body detection device 1 to which an ultrasonic sensor is applied includes: an intensity calculation unit for processing a reflection wave from a person 10 and calculating a reflection intensity according to a distance from the sensor； an intensity change calculation unit for calculating a temporal change in the intensity； and an ultrasonic sensor 100 for transmitting reflecting site information in which the distance, the reflection intensity, and its temporal change are combined, to a device body 200. The device body 200 includes a determination unit for determining whether the person 10 is in a normal rest state or in a dangerous respiration stop state, by referring to the temporal change in the reflection intensity of the person 10 from the change in the reflection intensity and the reflecting site information, and if the person is determined to be in the dangerous state, a notification unit immediately executes processing to notify the outside.SELECTED DRAWING: Figure 1
