1. An apparatus for processing an ultrasonic signal, comprising: a signal processing circuit configured to calculate integrated backscatter values by extracting radio-frequency signals in predetermined time regions respectively from a plurality of radio-frequency signals which are produced when an examinee is scanned by an ultrasonic wave; the signal processing circuit being configured to calculate a variance value of the integrated backscatter values; wherein the examinee is a plaque in a blood vessel; and the signal processing circuit is further configured to compare the variance value with a predetermined threshold value and to judge that the examinee is a lipid rich plaque when the variance value is greater than the predetermined threshold value.