A moving object detection device using an ultrasonic sensor, a method thereof, and a warning system using the same is provided. The moving object detection device includes: an ultrasonic sensor configured to receive a plurality of reflected signals per period; and a detector configured to divide a sensing distance of the ultrasonic sensor into a plurality of regions, detect a region of the plurality of regions as a region of interest (ROI) when an intensity of a reflected signal of the plurality of reflected signals exceeds a predetermined threshold in a region of the plurality of regions, and detect a moving object based on changes in the ROI.