Components of an ultrasonic sensor are contained in a cylindrical casing having an end wall closing one end of the casing. A vibrator such as a piezoelectric element is disposed in the casing in contact with the end wall. An integrated circuit chip that includes a signal generator, a filter device and a temperature sensor is contained in the casing. Vibrations generated in the vibrator are transferred to the end wall that transmits ultrasonic waves. Ultrasonic waves received by the end wall are converted into electrical signals in the vibrator. Frequency of the transmitting signals and a frequency region of the signals to be received are controlled according to the ambient temperature, so that they always coincide with a resonant frequency of the end wall which changes according to the ambient temperature. Thus, high transmitting/receiving efficiency is always realized, irrespective of the ambient temperature.