An ultrasonic sensor (1000, 1000A, 1000B), comprising: a piezoelectric element configured to transmit and receive an ultrasonic signal (4a, 4b), the piezoelectric element having a reverberation frequency that is variable based on a variation in an environment of the ultrasonic sensor (1000, 1000A, 1000B); a transmitter configured to drive the piezoelectric element; a burst signal sending circuit configured to deliver a burst signal (S0, S1, S2) to the transmitter, the burst signal having a burst wave frequency; a receiver configured to accept a reception signal received by the piezoelectric element; an amplification circuit configured to amplify the reception signal to output an amplified signal; an assessor configured to assess the amplified signal output from the amplification circuit; and an operator configured to detect at least one of the presence/absence of an object and the distance to the object based on the amplified signal which includes information on a reverberation time or the reverberation frequency of a reverberation signal; characterized in that the assessor is configured to assess the reverberation frequency of the reverberation signal contained in the amplified signal or a reverberation time that depends on a difference of the reverberation frequency from the burst wave frequency, and to feed a result of assessment back to the burst signal sending circuit, and based on the result of assessment, the burst signal sending circuit is operable to adjust the burst wave frequency, wherein the burst signal sending circuit is operable to automatically correct the burst wave frequency according to the variation of the reverberation frequency caused by the environment of the ultrasonic sensor (1000, 1000A, 1000B). a piezoelectric element configured to transmit and receive an ultrasonic signal (4a, 4b), the piezoelectric element having a reverberation frequency that is variable based on a variation in an environment of the ultrasonic sensor (1000, 1000A, 1000B); a transmitter configured to drive the piezoelectric element; a burst signal sending circuit configured to deliver a burst signal (S0, S1, S2) to the transmitter, the burst signal having a burst wave frequency; a receiver configured to accept a reception signal received by the piezoelectric element; an amplification circuit configured to amplify the reception signal to output an amplified signal; an assessor configured to assess the amplified signal output from the amplification circuit; and an operator configured to detect at least one of the presence/absence of an object and the distance to the object based on the amplified signal which includes information on a reverberation time or the reverberation frequency of a reverberation signal; characterized in that the assessor is configured to assess the reverberation frequency of the reverberation signal contained in the amplified signal or a reverberation time that depends on a difference of the reverberation frequency from the burst wave frequency, and to feed a result of assessment back to the burst signal sending circuit, and based on the result of assessment, the burst signal sending circuit is operable to adjust the burst wave frequency, wherein the burst signal sending circuit is operable to automatically correct the burst wave frequency according to the variation of the reverberation frequency caused by the environment of the ultrasonic sensor (1000, 1000A, 1000B).