An ultrasonic sensor (11, 12) comprising: a disk-shaped piezoelectric element that transmits and receives ultrasonic waves, and an acoustic-matching layer, with the piezoelectric element being housed in a case and fixed, characterized in that the piezoelectric element is {Li x (K 1-y Na y) 1-x} (Nb 1-z Sb z) O 3 having a composition including Bi (bismuth) and Fe (iron) of a metallic element as additives, satisfying the range of 0.03≦x≦0.045, 0.5≦y≦0.58, 0. 03≦z≦0.045, wherein the mechanical-quality factor Qm is 50 or less, the radial-frequency constant Np is 2800Hz·m or more, and the piezoelectric constant d 33 is 250pC/N or more, and wherein the acoustic-matching layer has an outer-diameter dimension equal to or greater than the outer-diameter dimension of the piezoelectric element; and further characterized in that when the radial-resonance frequency of the piezoelectric element is represented as f, when the acoustic velocity of a longitudinal wave propagating within the acoustic-matching layer is represented as c, when the wavelength of the ultrasonic wave propagating within the acoustic-matching layer is represented as λ, when the thickness of the acoustic-matching layer is represented as t, then the relationship of λ=c/f is satisfied, as well as t=λ/4±20%, and t/λ is between 0.210 and 0.242 or between 0.270 and 0.299, and characterized in that when a frequency range at the transmission/reception sensitivity that is reduced by 6dB from the maximum transmission/reception sensitivity is the frequency band, when a lower-limit frequency in the frequency band is the lower-limit frequency, when an upper-limit frequency in the frequency band is the upper-limit frequency, when an intermediate value between the lower-limit frequency and the upper-limit frequency is the center frequency fm, when the difference between the lower-limit frequency and the upper-limit frequency is the frequency bandwidth Δf, and when the ratio between the bandwidth Δf and the center-frequency fm is the fractional bandwidth Δf/fm, then the fractional bandwidth Δf/fm is 15% or more. a disk-shaped piezoelectric element that transmits and receives ultrasonic waves, and an acoustic-matching layer, with the piezoelectric element being housed in a case and fixed, characterized in that the piezoelectric element is {Li x (K 1-y Na y) 1-x} (Nb 1-z Sb z) O 3 having a composition including Bi (bismuth) and Fe (iron) of a metallic element as additives, satisfying the range of 0.03≦x≦0.045, 0.5≦y≦0.58, 0. 03≦z≦0.045, wherein the mechanical-quality factor Qm is 50 or less, the radial-frequency constant Np is 2800Hz·m or more, and the piezoelectric constant d 33 is 250pC/N or more, and wherein the acoustic-matching layer has an outer-diameter dimension equal to or greater than the outer-diameter dimension of the piezoelectric element; and further characterized in that when the radial-resonance frequency of the piezoelectric element is represented as f, when the acoustic velocity of a longitudinal wave propagating within the acoustic-matching layer is represented as c, when the wavelength of the ultrasonic wave propagating within the acoustic-matching layer is represented as λ, when the thickness of the acoustic-matching layer is represented as t, then the relationship of λ=c/f is satisfied, as well as t=λ/4±20%, and t/λ is between 0.210 and 0.242 or between 0.270 and 0.299, and characterized in that when a frequency range at the transmission/reception sensitivity that is reduced by 6dB from the maximum transmission/reception sensitivity is the frequency band, when a lower-limit frequency in the frequency band is the lower-limit frequency, when an upper-limit frequency in the frequency band is the upper-limit frequency, when an intermediate value between the lower-limit frequency and the upper-limit frequency is the center frequency fm, when the difference between the lower-limit frequency and the upper-limit frequency is the frequency bandwidth Δf, and when the ratio between the bandwidth Δf and the center-frequency fm is the fractional bandwidth Δf/fm, then the fractional bandwidth Δf/fm is 15% or more.