An ultrasonic distance sensor generates ultrasonic waves and transmit same toward a target surface, the distance to which is to be measured, through a predetermined path. The sensor is also provided with a reference surface which reflects ultrasonic waves, which reference surface is disposed within the aforementioned predetermined path of the ultrasonic waves. A receiver associated with delayed pulse generators receives the ultrasonic waves reflected by the reference surface or by the target surface, selectively. Arithmetic operations based on the elapsed time between transmission of the ultrasonic waves and reception of the ultrasonic waves from the reference surface derive the transmission velocity of the ultrasonic wave based on the known distance to the reference surface. On the basis of the derived transmission velocity and the elapsed time measured with respect to the ultrasonic waves reflected by the target surface, the distance to the target surface is derived. The aforementioned ultrasonic sensor is used to monitor the external shape of an object. In order to monitor the external shape of the object, a system is composed of a plurality of the ultrasonic sensors. The sensors comprises a primary sensor or sensors directed toward the external surface of the object and an auxiliary sensor or sensors directed to a reference line.