An ultrasonic sensor comprising: a piezoelectric element; the piezoelectric element being provided with a flat-plate-shaped piezoelectric body that includes a transmission region that is for transmitting an ultrasonic wave in response to having an alternating-current voltage applied thereto and a reception region that is for receiving a reflected wave of the ultrasonic wave, a common electrode that is provided in the transmission region and the reception region, a transmission electrode that is arranged so as to face the common electrode with the transmission region interposed therebetween and that is provided in the transmission region, and a reception electrode that is arranged so as to face the common electrode with the reception region interposed therebetween and that is provided in the reception region; a semiconductor element that is electrically connected to the transmission electrode and the reception electrode and that is configured to switch an electrical path extending between the transmission electrode and the reception electrode between a conductive state and a non-conductive state; and a phase adjuster electrically connected to the semiconductor element along the electrical path extending between the transmission electrode and the reception electrode; wherein the semiconductor element is configured to put the path into the conductive state after application of the alternating-current voltage is stopped, and as a result, a reverberation signal output from the reception region in accordance with a reverberant vibration of the ultrasonic wave is fed back to the transmission electrode; and the phase adjuster being arranged to adjust the phase of the reverberation signal fed back to the transmission electrode such that the reverberant vibration can be suppressed. a piezoelectric element; the piezoelectric element being provided with a flat-plate-shaped piezoelectric body that includes a transmission region that is for transmitting an ultrasonic wave in response to having an alternating-current voltage applied thereto and a reception region that is for receiving a reflected wave of the ultrasonic wave, a common electrode that is provided in the transmission region and the reception region, a transmission electrode that is arranged so as to face the common electrode with the transmission region interposed therebetween and that is provided in the transmission region, and a reception electrode that is arranged so as to face the common electrode with the reception region interposed therebetween and that is provided in the reception region; a flat-plate-shaped piezoelectric body that includes a transmission region that is for transmitting an ultrasonic wave in response to having an alternating-current voltage applied thereto and a reception region that is for receiving a reflected wave of the ultrasonic wave, a common electrode that is provided in the transmission region and the reception region, a transmission electrode that is arranged so as to face the common electrode with the transmission region interposed therebetween and that is provided in the transmission region, and a reception electrode that is arranged so as to face the common electrode with the reception region interposed therebetween and that is provided in the reception region; a semiconductor element that is electrically connected to the transmission electrode and the reception electrode and that is configured to switch an electrical path extending between the transmission electrode and the reception electrode between a conductive state and a non-conductive state; and a phase adjuster electrically connected to the semiconductor element along the electrical path extending between the transmission electrode and the reception electrode; wherein the semiconductor element is configured to put the path into the conductive state after application of the alternating-current voltage is stopped, and as a result, a reverberation signal output from the reception region in accordance with a reverberant vibration of the ultrasonic wave is fed back to the transmission electrode; and the phase adjuster being arranged to adjust the phase of the reverberation signal fed back to the transmission electrode such that the reverberant vibration can be suppressed.