An ultrasonic sensor is provided at a bumper 31 of a vehicle 3 and used in a vehicle obstacle detector 1 that detects the presence of an obstacle within a detection area by transmitting and receiving ultrasonic waves. The ultrasonic sensor includes a piezoelectric element 21 configured to transmit and receive the ultrasonic waves by using vibrations of a piezoelectric substrate and a plate-shaped acoustic matching layer 22 on one surface 22a of which the piezoelectric element is attached. The ultrasonic sensor is inserted in a mounting hole formed through the mounting surface. Another surface 22b of the acoustic matching layer 22 opposite to the one surface 22a is inclined with respect to the one surface 22a on which the piezoelectric element is attached, and is formed to be parallel with the outer surface of the bumper 31.