1. An ultrasonic imaging apparatus comprising: a transmitter confiqured to transmit ultrasonic waves directed to an object; a receiver configured to receive ultrasonic waves coming from the object, and to output signals representing the received ultrasonic waves to the processor; and a processor configured to process signals received from the receiver, and to generate an image a including plurality of frames; wherein the processor is configured to: assume one frame as a reference frame, in the image including a plurality of frames being generated, set a region of interest at either of a predetermined position and a position accepted from an operator, assume another one frame as a comparative frame, set a search area wider than the region of interest, at either of a predetermined position and a position accepted from an operator, set multiple candidate regions in the search area, which are destination candidates of the region of interest, calculate a degree of similarity between an image characteristic value in the region of interest and an image characteristic value in the candidate region, with respect to each of the candidate regions, so as to obtain a similarity distribution of the degrees of similarity across the search area; and obtain statistics that compare a minimum value of the degree of similarity with an overall value of the degree of similarity in the similarity distribution, by using the minimum value of the degree of similarity, a mean value of the degree of similarity, and a standard deviation of the degree of similarity, and compares the obtained statistics with a threshold value, thereby determining the confidence of the region of interest.