An ultrasonic sensor (1,21), comprising: a closed-end tubular casing having a vibration area at a bottom surface; a piezoelectric device disposed in an opening of the casing and attached to the vibration area; a terminal portion having an end portion that is disposed in the opening of the casing, the terminal portion extending from inside the opening of the casing to outside the opening of the casing; and a vibration damper (6, 7, 8, 16, 17, 18) mainly containing a mixture of inorganic filler and a polyol-based polyurethane, and inserted into the opening of the casing, wherein the vibration damper (6, 7, 8, 16, 17, 18) having: a glass-transition temperature lower than the lower limit temperature of an actual-use temperature range, and a loss modulus versus temperature characteristic having a peak centered at the glass transition temperature, wherein a point of inflection located on a high-temperature side of the peak is out of the actual-use temperature range; wherein the vibration damper (6, 7, 8, 16, 17, 18) has a loss modulus exceeding 0.2 Mpa at room temperature of 25°C wherein the vibration damper (6, 7, 8, 16, 17, 18) has a loss modulus below 53.5 Mpa throughout the entire actual-use temperature range; wherein the vibration damper (6, 7, 8, 16, 17, 18) has a loss modulus versus temperature characteristic in which a temperature range corresponding to a width of 3 dB at the peak lies below the lower limit temperature of the actual-use temperature range, the peak having the glass-transition temperature at the center; wherein the vibration damper (6, 7, 8, 16, 17, 18) has a loss modulus exceeding 1.5 Mpa at the lower limit temperature of the actual-use temperature range; and wherein the actual use temperature range is between -40°C and 85°C. a closed-end tubular casing having a vibration area at a bottom surface; a piezoelectric device disposed in an opening of the casing and attached to the vibration area; a terminal portion having an end portion that is disposed in the opening of the casing, the terminal portion extending from inside the opening of the casing to outside the opening of the casing; and a vibration damper (6, 7, 8, 16, 17, 18) mainly containing a mixture of inorganic filler and a polyol-based polyurethane, and inserted into the opening of the casing, wherein the vibration damper (6, 7, 8, 16, 17, 18) having: a glass-transition temperature lower than the lower limit temperature of an actual-use temperature range, and a loss modulus versus temperature characteristic having a peak centered at the glass transition temperature, wherein a point of inflection located on a high-temperature side of the peak is out of the actual-use temperature range; a glass-transition temperature lower than the lower limit temperature of an actual-use temperature range, and a loss modulus versus temperature characteristic having a peak centered at the glass transition temperature, wherein a point of inflection located on a high-temperature side of the peak is out of the actual-use temperature range; wherein the vibration damper (6, 7, 8, 16, 17, 18) has a loss modulus exceeding 0.2 Mpa at room temperature of 25°C wherein the vibration damper (6, 7, 8, 16, 17, 18) has a loss modulus below 53.5 Mpa throughout the entire actual-use temperature range; wherein the vibration damper (6, 7, 8, 16, 17, 18) has a loss modulus versus temperature characteristic in which a temperature range corresponding to a width of 3 dB at the peak lies below the lower limit temperature of the actual-use temperature range, the peak having the glass-transition temperature at the center; wherein the vibration damper (6, 7, 8, 16, 17, 18) has a loss modulus exceeding 1.5 Mpa at the lower limit temperature of the actual-use temperature range; and wherein the actual use temperature range is between -40°C and 85°C.