An ultrasonic sensor provided at a flip-up back door that opens and closes via a pivot shaft provided at a rear top of a vehicle, the ultrasonic sensor detecting an object in a vicinity of the vehicle by intermittently transmitting an ultrasonic wave, which hits the object to thereby generate a reflected wave as a reflection of the ultrasonic wave, the ultrasonic sensor including a sensor head provided at the back door and including a transmitter transmitting the ultrasonic wave and a receiver receiving the reflected wave, wherein the sensor head is provided at a position in a vicinity of a centerline of the vehicle in a width direction thereof and where a distance from a corner of an edge portion of the back door to the sensor head is shorter than a distance from the sensor head to a small aboveground structure provided on the ground.