An ultrasonic sensor for detecting an object includes a sending element for sending an ultrasonic wave to the object, a receiving portion for receiving the wave reflected by the object, a first transmitting portion for transmitting a vibration of the wave received by the receiving portion, a second transmitting portion and an oscillating portion which is oscillated by the vibration. The receiving portion is exposed to a space where the object exists. The second transmitting portion is coupled between the first transmitting portion and the oscillating portion so as to transmit the vibration from the first transmitting portion to the oscillating portion.