1. A method for estimating acoustic velocity of an ultrasonic image, comprising: (A) dividing each of the ultrasonic images of a plurality of input frames into a plurality of blocks; (B) extracting contours of ultrasonic images corresponding to each block divided into a plurality of blocks for one frame among the ultrasonic images of the plurality of divided frames; (C) calculating and analyzing average luminance values of each block in order to select the optimal blocks in a sequence of blocks having the maximum luminance value among average luminance values of each block when the contour extraction for each block is completed; (D) determining the optimal block number according to the average luminance of the analyzed frames by calculating the average luminance values of the analyzed frames using the average luminance values of each analyzed block and selecting the optimal blocks as many as the optimal block number; and (E) selecting, as the optimal frame, the frame corresponding to the optimal blocks having the maximum luminance value among the optimal blocks of each frame after comparing the selected optimal blocks with the optimal blocks of the remaining frames and estimating and applying acoustic velocity of the optimal frame as the real acoustic velocity.